iPhone 4 won't sync with itunes properly?

A few weeks ago I had to completely wipe my phone and put everything back on it because when i plugged my phone into iTunes, it would say that I have no songs on my phone however there were over a thousand on it.

Just recently, I have set it up so that there is a playlist of songs on my computer, and my iPhone syncs to that playlist. However, I have come up with the same problem as before; When I plug my phone into iTunes, the computer shows that there are no songs on my phone, however again there are over 1000 on it. The computer also won't let me sync my phone with iTunes. Whenever i plug it in, a message comes up saying that "there isn't enough additional free space on the phone, an additional 7GBs is required"

I looked this up online already, and the only thing that i have set to sync automatically are the 1000 songs, so I'm not really sure whats going on. Please help!