What is wrong with my iTunes account?

I had a gateway computer and had made an iTunes account on it. Like any iTunes account, it showed all the movies, music, apps, etc. that i purchased. I recently bought an iMac and when i went to go on iTunes, it asked me to log into my account so i logged in with my apple username and password and it logged in successfully but it didn't show any of the songs, apps, etc. that i downloaded synced on my iTunes account. The only thing it showed was 8 songs that i never downloaded and never heard of. I know it is my account because i logged in and out multiple times without success. Just to make sure it didn't erase all of my data, i checked back on my gateway and all my information, songs, apps, etc. was there. Please help me get my account on my new iMac.