Is this water retention? if so, how do i avoid it?

ok, so when i wake up in the morning, my stomach mostly flat (i think its cuz i drink green tea...and some other things that have changed. it used to be like this, then it got screwed up, then back to normal again.) but as soon as i take that first bite of food in the morning, bam! my stomach starts to expand. it does this for the whole day, no matter what i do...it didn't used to do this. or at least, not as badly. and my eating habits haven't really changed. then i got indegestion from taking PrevAcid for too long, which, in combination with my birth control (i think) really screwed me up...i'd wake up with most of the fluff i went to bed with, and it just got worse and worse. :(. anyway, when i wake up, its mostly gone, so how do i get rid of "fluff" that's gone the next morning, reappears, etc? i don't even know what's causing it....is it fluid retention? like i said, it never used to do this...i'm still a teenager, and my eating habits haven't really changed. i exercise regularly. i look great when i wake up, but it never lasts! what can i do to prevent this (starting right when i wake up?) every day after breakfast, i drink green tea. every day. help?