Help! Baby suddenly fighting bottle!

My daughter will be 3 months on the 29th and is formula fed. She takes Zantac for reflux. In the past few days she's started giving us a hard time taking the bottle. She cries and cries and I know she's hungry. I have to put the nipple in her mouth and move it around for awhile before she finally latches on. Then when I stop halfway to burp her we have to start the process all over again. I have no idea why she's doing this all of a sudden and I don't know what to do about it!?