But you really need to, you took the responsibility to have sex now you should be responsible enough to go in and buy a test. And to be honest its not that big of a deal. Go to wal mart and just go thru the check out and do it, they won't say anything to you, or have your girlfriend do it, its not as bad as you think.
And its better to be safe and know whats going on with her than to wonder all the time.