Stuff added to acc?

Hey guys tell me something if you can, i have an acc that i wasnt using, was focoused on reaching this acc to gold yet i have been getting a lot of flamers so i decided to pick up the acc i used to play for fun only and see if i could climb faster due to the fact that after placements the lp you gain is awsome, it happens that the acc had way more champs and way more skins than it had since the last time i used it so my guess is that someone was playing on that acc, for protection i changed the password now my question is, if someone was playing on my acc, and they see i changed the password can they do something to get it back? im asking because if someone was using my acc then the ip that will show up in majority will be theirs, the computer that im using is the same i was using before i "leave" the acc so my computer id should stay the same. Thank you all for your time.