It sounds like it could be TT. FWIW, my DS did this around the same age. I stopped feeding him on my right as he struggled to latch there. I blamed it on my flat nipple. Turns out he had a posterior tongue tie and upper lip tie which made it hard to latch on that side. We just feed on the left now. Do you have any bf support groups you can get to?
Could it be possible she is straining to draw the milk out of that breast for some reason?
My DD favours one breast over another and I think it is because there is more milk in the one she favours. With the other one, she squeezes and pulls on it to get the milk out toward the middle and end of a feed.
My 16 week old has fed well pretty much since birth and after a week or so of using lanisoh cream whilst my nipples got used to things, bfing has been pain free. However in the last week she has been behaving strangely when feeding from my right breast. She will suddenly do a massive squirming movement whilst feeding, pulling her head (and my nipple!) down and away. She doesn't do this during every feed but when she does do it it's often several times in a row. She stays latched on and carries on feeding afterwards. I've also noticed that her latch on that side has got a bit lazy and my nipple is getting sore and sometimes look lipstick shaped after a feed. I don't think it can be tongue tie though at this late stage can it? She can stick her tongue out a long way. Other information that may be relevant: she is a bit refluxy although this is getting better. I can't see any signs of thrush. Any advice/ suggestions very welcome.