it says i tweeted but i didnt?

[Agent G!]

well i hadn't been on twitter for like over 2 weeks and then my friends says she talked to me on there and i didnt reply,she's on twitter alot ! anyway,i went on twitter and i noticed that it said i tweeted like 300 or something like that tweets,i dont tweet alot,anyway ! i went and checked them they were like these marketing things! saying like watches and stuff 15 % off real high quality and see who's stalking u and eager to lose weight..go to blah blah blah ,i told my friend and she said to change my password so i did ! but i dont know..will this happen again? is there a virus in my account or something like that??the last i check the last tweet was 2 hour before i logged in and before that 8 and before that 6 and so on..... so help me please ! what is this?