she won't breastfeed anymore?

My baby only breast feeds sometimes but it's rare, she just started doing this about two weeks ago. I've been struggling to get her to breastfeed. She latches on but after a couple minutes sometimes seconds she matches off. I don't know if it's a lot of milk flow or not enough? I never had this problem before and I can't figure it what the problem is. I've been having to give her  formula when I'm not able to pump. She will take a bottle just fine but not my breast. I squeeze gently to see if I'm having milk come out and I am, so what could the issue be? Please if someone could help!