It sounds like your little 2.5 gallon is cycling. I'm assuming it has a filter? If not, that's really weird, haha.
Keep watching the levels and doing water changes like you are and wait for the levels to balance out. It'll probably take 2-3, sometimes 4 water changes a week. But it should balance out in a week or two. Since it's a smaller tank, you'll always have to watch the levels, but after it balances out you should be able to go back to 1 75% change a week.
For your water changes, start just vacuuming (you can leave your fish in there), instead of removing/rinsing. That's going to make it take longer to cycle. It probably won't be a consistent cycle in a tank that size, which is why you'll always have to keep an eye on the ammonia and nitrite.