Undo'ing Purchases in Shop Reduces Current HP Drastically

I was tooling around with some items in the shop the yesterday, and hit Undo to revert a purchase. Suddenly my HP went from 100% to about 20%. I started with over 50%. I was not buying and selling HP items, either. I was playing Malzahar, had Catalyst the Protector, and was buying a Blasting Wand, then returning it. Super weird.