thanks for the support guys and gals.
she had been lulling around the bottom of the tank a lot for the last 2 or 3 days and only came up for air and food (she still had an appetite!). it didn't occur to me that she may have been sick as most of my females like being lazy, but they tend to stay at the top not the bottom. she had a normal looking body, no swelling, etc., except for the yellow belly. i'm guessing they were a sort of parasite or something but i didn't have the nerves to dissect her.
i've had them for 6 months or so and at first i bought 5 but now i only have 3 left (the first one got stuck in between rocks and died). the last fish were added at the latest 4 months ago so i don't think it has anything to do with new fish bringing in diseases.
sorry i don't know how to quote replies so i had to answer questions randomly.