Baby throws up every time after bottle, but not breast

Our baby is 6 weeks old and is feeding every 0.5-2 hours. Most of the time she nurses with mom but I try to help out every once in a while with some pumped milk. Over the past few days I’ve bottle fed her 6-7 times and every time she spits up. Not just a trickle from the corner of her mouth, but milk coming out of her nose and mouth and it seems like a lot. She doesn’t cry so I don’t think she’s in pain and is usually fine after. She doesn’t appear to have issues with the vomiting overall and is gaining weight like a champ. And she never vomited this much in the first few weeks despite eating more in one go and eating faster. 

I’ve tried feeding her on an incline, feeding her less and less (the last throw up was with 40mL, which is less than 1.5oz), taking longer and longer to feed her (for example 20 minutes to feed her just 2 oz), burping her in an upright position afterwards for 20 minutes, holding her upright for 20 min instead of burping. Nothing works. And to make things worse she fusses and cries to keep eating on the bottle until she does throw up (I’ll try to rock her for 5 minutes after feeding an ounce to see if the fussing goes away but she cries harder, so I have to feed her more). It’s gotten to the point where I’m afraid to feed her. 

But she’s absolutely fine when breast fed. No vomiting when burping, laying flat when breastfeeding, etc.

Any ideas as to what’s going on? I have 2 guesses but would like to hear others’ experiences.

1.	 When I give her the bottle her stomach can’t handle the extra food. But if that’s the case why doesn’t the constant breastfeeding cause her to vomit? Isn’t she getting milk constantly from boobs too?
2.	The bottle nipple lets out too much milk. But we cycle through 3 different bottles including a Dr. Brown bottle and she throws up with every one of them. So it can’t be the nipple right?