4 mo fighting eating

My 4 month old Luvs drink warm milk but I noticed the last couple of weeks she fights to eat even her morning bottle she usually drinks about 4 to 5 ounces every 2 hours and she cries and cries and then eventually drinks a whole bottle but she struggles he's already on reflux medication seems like she takes a drink and then stops and cries not sure if it's Stephen I'm using a number 2 nipple from dr. Brown I just hope she's not in pain we switched her formula several times and now she's on Gerber Soothe