Ipod Touch Purchased Enquiry

I have an ipod TOuch, and on the ipod itself i have some how created 2 purchased accounts, one is just "purchased" and one is "purchased on sam"( name of ipod). I must state that i always buy all my music the same way via the itune app on the i touch.
I have just had my computer rebuilt as it got fried and stupidly did not back up all my files. I have lost all my music from disc which i understand, but do not understand why i lost all the music i "purchased on sam" when i have tried to resync my ipod for the first time, since the rebuild.
Thankyou for time