When i was playing normal game and went to shop, I lost a big amount of hp when i undo what i bought. Once it almost killed me and I had 10 hp after that happened. It does not happen every time and I noticed it does not happen at all when I buy homeguard 
upgrade on boots. Please help with that.
Not a bug; undoing reverts your champion state to what it was before you undid. So if you started with almost no health, you'll have almost no health again.
Otherwise, people could do cheesy things like: Buy Avarice blade while healing for the extra gold per second, then undo just before you walk out of base. Buy a Warmog's and undo, healing almost instantly(as opposed to having to regen slowly) Buy a bazillion AP as GP, heal yourself instantly, undo it all and buy your AD items.
Healing instantly isn't that big a deal, but in a competitive game, when you're defending your base, it could make a pretty big difference. So you can't.
Homeguards specifically cannot be undone, since it has an immediate effect on the game(you get lots of ms and instantly heal) So it doesn't have the "bug".
It's not what you think it is, what is actually happening is that when you undo, it undoes your health regen that you've regenerated while sitting in base before you bought the item. So if you were at full health, it will not do anything. It's a bug 
Did the item you undo have health or health regen on it?
the undo buton kinda works like a "reverse time" button.
