Thanks for the advice guys.
It could all be an innocent misunderstanding, or I'm being played. Even though our relationship is still relatively strong and we have a great time together, something just isn't sitting right with me at the minute, and when you feel like that, something is usually up.
She'll be coming over tonight after she finishes work. I'm going to gently confront her about it, but if she doesn't give me a good explanation, I'm going to finish it there and then. I've had plenty of chances to cheat if I wanted to, but didn't, because I wouldn't want it to happen to me.
Without sounding like a d**k, I'm a nice, good looking, intelligent lad, and don't deserve to be played like a fool. Men ringing your girlfriend while your in bed with her just isn't on. She should have apologized there and then and told me what the situation was. If she has nothing to hide, then why try to conceal it?
As I said, she either gives me a great explanation tonight or she's out of my life for good.