iTunes keeps wiping my iPhone's music?

Hi there.

I just bought two new albums today, so I imported them to iTunes as normal.

Tried putting them onto my iPhone, and it was going fine until I tried putting on the second album. Nothing happened, so I clicked and dragged again, and then it said (Updating files 1 of ... around 900? It was re-uploading all the songs I had on my phone for some unknown reason)

Then it came up with an error saying it couldn't sync around 450 songs because there wasn't enough memory left.

So I deleted all the music from my phone, and tried putting it all back on again. Got virtually all of it on, but when I tried playing a certain 4 track EP on my iPhone, it suddenly disappeared and came up "No content". iTunes then started syncing all 800+ songs again. I cancelled that, as it takes over 30 minutes each time, so now I only have about 20 songs on my iPhone.

I don't understand why this is messing up or how to fix it, I've never had this problem before.

Also, when I was transferring the music onto my iPhone, iTunes would automatically transfer purchased songs I didn't even select.

Any help would be appreciated?