My iTunes and iPhone are erasing my songs?

I have an iPhone 4S and my one laptop is the only thing I've ever sync'd it with. All of the current updates are on both my phone and my iTunes program.

A week or so ago I imported 5 CDs of music from one of my school classes into my iTunes account from my pc, then sync'd my phone with my iTunes to put the songs on the phone, which is what I wanted. It worked, great. Last night I was charging my phone while doing homework and it sync'd automatically, ok whatever it's updated now as well as charged right? After class today I tried to pull up the songs to finish an assignment with a classmate, and all 5 CDs worth of songs were gone, as well as a bunch of CDs and singles that I had purchased off of iTunes... When I got home from class I decided to get on iTunes and put those imported songs back onto my iphone, but they were gone from my computer too!

After tinkering with iTunes on my pc, I've gotten some of the songs back, but not all. There is one particular song that I want back on my phone from iTunes and when I go into the iTunes app on my phone it says the song is purchased, but the button is greyed out and says purchased so it does not allow me to download it. When I go into the iTunes store on my laptop, it says the same thing. Purchased, but I am not allowed to redownload it for some reason... I do not know how many of the iTune songs I have not noticed missing, but I am going to have to import all of the songs from my CDs back onto my pc and then put them back on my phone for class, it would be nice to fix this so I don't lose them again... iTunes removes songs off my iPhone all the time, it gets insanely annoying, but I've never noticed it remove them from the library itself before. Usually I just 'select all' and drag the songs onto the iphone before deleting the dups. I also try to never plug my phone into my pc because iTunes always gives me and my phone issues.

Any ideas?
Gibbs.