Can't move items around in inventory.

The game doesn't let me switch the items in my inventory. For example, if I bought zhonya's and wanted to put it on T instead of 5, it doesn't let me drag it from the 5 slot to the T slot. If I just click on it, it still activates, but for some reason, I can't move the item. I noticed it for the first time today in the middle of a game and I'm not sure what caused it, but since then, I haven't been able to move any items. Looked all over for a solution but can't find one. Anyone ever had this issue?