My htc is acting strange? please help.?

i recently (2 days ago) bought a second hand phone shop. and i had to give the man my name and postcode. basically, ever since i got it (htc mozart running windows phone 7) it has been acting strange as in when i put it down, it does lots of different things without me even touching it. like... calling someone, going out facebook, looking through contacts... and i have no idea why this is happening, is it a fault in the device? or is it someone that has hacked into it (guys in shop) going through my phone? i have no idea what is going on will someone please help me. thanks.