Syncing with iTunes quits halfway through iPhone 3G

I recently bought some music directly from my iPhone. When I went to sync it to my desktop iTunes, there were no problems going from the phone to the desktop. However, then the new music wouldnt go back into my library. Its not in my library and every time I try to sync it, I get an error code of iPhone is disconnected or some other random disk is not inserted message. Never had this issue before the most recent update. Why is it that I buy the music ON MY PHONE but its not IN MY PHONE iTUNES and I have to go to the desktop and then back to the phone? And more importantly, why is it quitting the sync mid-process, right before the music starts syncing? (it goes through the first few streps including syncing apps but quits right at the music).
HP iPhone 3gs