All the strings on my electric guitar went sharp a whole step by themselves??

i have an ibanaez sas32 with a tremolo system. this thing never goes out of tune and i mean never, not even when i use the whammy bar. i played it this morning and it was fine but when i got home today (7 hrs later) all the strings were sharpened by atleast one whole step. nobody touched it b/c no one was home and the temperature in my room is always the same. what could of caused this???