Why does my baby cry and fuss during breastfeeding?

Some moms notice their baby has been breastfeeding well in the past, but suddenly become fussy, they seem to reject breastfeeding, and that leaves you wondering what's going on. It can be due to a lot of things. It's easier to get to the root cause when you think about when your baby is fussy. Is it at the beginning of a feeding? Is it before or after let-down? Is it at the end of a feeding session? Different problems will happen at different times. Maybe there's too much tissue in the baby's mouth or it's going too far back, so that's causing them to gag. If that's the case, then let the baby naturally latch on by gently angling the nipple up towards the top of their mouth. Once it touches the top, that's what stimulates them to suck and latch on.
If your baby is fussy after you've latched them on, but before your milk lets down, maybe it's because they're impatient and want it to come faster. A baby usually has to suck for 30-60 seconds in order for milk to let down. Some babies become lazy suckers and want it to come faster. These babies do well with bottles, but not well on the breast, because they get instant flow with bottles. So what you can do is pump for a couple minutes before you latch your baby on. That will get the milk coming faster. Then latch your baby on before you have a let-down. That eliminates the period of time they have to wait.
Sometimes a baby is fussy after your milk lets down because it's a very forceful or quick flow, and they're having a hard time keeping up with it, so they get fussy and start to choke and gag. That can cause them to take in a lot of gas, so that can cause issues a couple minutes down the road. They may not eat well for the rest of the feeding because their tummy is full of gas. If this seems to be the problem, then try burping them. For forceful let-downs, you can try pumping a bit of milk off first and then latch your baby on. It's the opposite problem of them being impatient; it's coming too quickly. So pump a bit off first, then latch them on and see if that makes a difference. Along with burping, they should be burped after each side and then after the entire feeding. Babies may not always burp, but it's good to give them the chance to try.
The time of day may have something to do with it too. If your baby is more fussy and seems to resist breastfeeding more in the evening, that's actually pretty normal. A lot of babies are more fussy in the evening. Sometimes it comes down to something that the mother is eating at a certain time each day, so it's in the mother's milk at a certain time each day. If your baby is fussy not just during feedings, but in between and all the time, it might be your diet. You could try cutting out gas-inducing foods, caffeine, chocolate, dairy products; sometimes those things make a difference. Pick one thing and cut them out of your diet one at a time to see if you can nail down what's causing your baby issues. If that doesn't work, call your pediatrician. They'll be able to give you tailored advice on what you need to do.
Another reason babies get fussy at the end of feedings is simply because they're done, or they're not wanting to eat; they're just wanting to suck. So if you take your baby off the breast, burp them, and give them a pacifier, then maybe they'll be perfectly content. In other situations, babies aren't getting enough to eat, so they're continually fussy after a feeding. If you suspect this is an issue, there are things you can do to build up a milk supply and see if it makes a difference.
Another possibility is that your baby is going through a growth spurt. They'll naturally be a bit more fussy, want to eat a bit more, and this is how your body is stimulated to make more milk over time. So in the first 4-6 weeks of your baby's life, you make 2-3oz every 2-3hrs or so. By the time the baby is 6 months old, you may be making as much as 6-8oz at different times of the day. This increase happens naturally as babies experience growth spurts. Growth spurts typically happen around 7-10 days, 2-3 weeks, 4-6 weeks, and then 3, 6, and 9 months. So if your baby is extra fussy around these times, maybe that's the problem.
As babies get older and start to develop more, they become more aware of their surroundings. Sometimes the issue is distraction. So nurse in a quiet room, turn off the TV, dim the lights, turn on some peaceful music, and see if that makes a difference. At the end of the day, the most important thing is that your baby is getting enough to eat, so watch for signs that this is happening. If your baby is having at least 6 wet diapers a day and regular bowel movements, then chances are things are going well. You should check with your pediatrician, track their growth, and as long as they're gaining the appropriate amount of weight, then you're making enough, and other issues for fussiness can be looked into.