Your milk might be coming in too fast and that may make her take in more air, which can make her fussy, spit up more and can make her feel full sooner, so she's hungry sooner.
I had this "problem" with both my boys. Try and put one hand on top of your breast (about 2 fingers above her mouth) and push to slow down the milk. When your breast has been drained you can slowly let go and it won't come out so fast.
Burping her more often during the feed can also help.
Domperidone does go into the milk in very small amounts so side effects in babies are not expected. The amount they get is much smaller then the amount they are prescribed for treatment of reflux, so I don't think that's what makes her fussy.
The drug is normally used to help the tomach to empty more quickly. (It helps to reduce reflux and the sensation of feeling full)as a side effect it increases milk supply because it increases the secretion of prolactin, which is needed for milk supply.
You can read more about Domperiodone here:
http://www.breastfeedingonline.com/domperidone.shtmlHope this helps! I really think she's fussy because she has a hard time to handle the amount of milk coming in...
I have heard of women stop taking domeridone and the body took over the milk supply, but talk to your doctor first about that.