HELP!! Feeding question

So recently we have started giving Alison a little bit of formula every now and then. It started as 3oz breastmilk and 1oz formula just because I was only making 3oz at a time and she was eating 4. Then it was half and half, then became convieninet and I fed her just formula a couple times a day. Now I'm noticing that after breastmilk feedings she is freaking miserable! She does great on the formula, but when I feed her my milk now she seems gassy and starts screaming while trying to poo. And nothing I do helps. She curls her feet up to her chest and grunts like she is pushing super hard then starts screaming. And before this the RARELY cried. She would only cry when getting shots or an iv put in. Now she screams bloody murder like she is in sooo much pain and it breaks my heart. I had every intention of breast feeding her as long as possible. I was going to try for a year. But now it seems like my breast milk is upsetting her. Could it be? Did feeding her formula ruin her on breast milk? I have never heard of something like that happening. I'm going to ask her Dr next friday, but until then do you think I should experiment with it and do a mixture again? Or just go with formula? I would hate to stop breast feeding all together.

It could be something you've eatten...like caffeine or chocolate can really upset a baby's belly. I would mix. That's what I did when I wasn't pumping enough.

Is she screaming while eating or just after. Maybe the formula is causing it if she was never having any problems before? Once I gave my son formula he would literally gag and throw up breastmilk whether from the tap or bottle.

She screams like an hour after. She will be fine on formula. I gave her formula all day yesterday and she was great. But I gave her breastmilk this morning and an hour later she was miserable. It looked like she was trying to poo and she was grunting and pushing then she would scream and cry. Then at her next feeding I gave her formula, now she is sleeping 'like a baby'.