He wiggles when BF - not sure why. It hurts though

[empen]

At his 7pm feed he has started to wiggle and thrash around. Really poor feed. latches, sucks a bit, thrashes and tries to latch again, snuffling. It is really uncomfortable. He does it on both breasts.
In the end I have been giving him a couple of oz of formula but then he is sick quite a lot.
Anyone had any experience or any reasons why he may have started doing this recently?

how old is he?
my 12 week old does this sometimes. more so when he is tired and is feeding for comfort rather than milk. sometimes I have to take him off, settle him down a bit and then try again.
if he's hungry he'll eventually feed nicely, if not he goes off to sleep