Your supply would not get permanently ruined from 2 days of supplementing but may if you do this daily and continue, what she takes in formula she is not taking from you so your body will say "hey, we don't need to make more, no one's drinking it!". When you say she took a little formula I think most babies will, especially bf'd babies- it's so easy for them to drink it. My dd would never turn down a boob, she would always drink some, hungry or not, she may not take a full feed if not hungry but she would take some. Have you tried to put her to 3 hours instead of 2.5? It may make her hungrier to take more at a feed. How many feeds is she getting and how many night feeds? If she has lots of night feeds she's loading up and not needing as much in the day.
I would not worry about not being as big as you once were. I never feel "full", only feel slightly full at my dd's 4am feed. The only times I EVER felt full was when my dd slept through the night w/o waking once (happened twice). How many wet diapers does she have in a day? This is a good indication of her feeding. Is she gaining weight? If you are concerned would it be possible for you to go to a bfing clinic nearby to get her weighed more often and to check latch? Again, the only way to increase supply (if you truly need to) is to let her suck more frequently temporarily and to pump. Doing this, it does not take long for the body to make more.