This happened in my inventory also. Basically what happened was I used an item from my med kit but it glitched and stayed in my inventory even after it wasn't there (just continued to say "Receiving..." when I right clicked it because it didn't really exist there anymore), and so there was REALLY one more free slot under it (because med kit is 2x2 slots), I then created a splint and because the med kit is 2x2 slots, it thought I only had one more free slot vertically, but really it had two (but the top slot was being used by the glitched item), so it still put the splint there and made the med kit glitch and have an extra slot downwards.