How to apologize to my girlfriend?

I really screwed up. I have been dating my girlfriend for almost 3 months. We have done a lot of sexual flirting, but just stupid stuff. She was on top of me and me were making out, I put my hand down her pants a couple of times and felt skin. Later on after I had to go home we started chatting on facebook. while we were chatting we mentioned the whole me touching her and we started joking around. I had a previous girlfriend that was a total whore. She was the definition of slut. I wasted 2 months on that slut and in the end she never really liked me in the first place. She broke up on me when I went for "second base" and moved my hand between her legs. She slapped me and ended it right there. You should also understand that I'm no "hot guy" either. I'm 6'2'' and 250 pounds. She was messing with me the whole time and now she has some funny story about how the fat kid went for second base with her to tell to her slutty cheer leading friends. My current girlfriend knows about that and while we we're talking about that I jokingly said "Well, it seemed you liked it more than my ex-girlfriend." She now thinks that I called her a slut. It was 1am, I was tired. As it is I have a terrible time putting the words together right, It being so late made it even harder. I've apologized several times. I've taken full responsibility for it. I tried calling her, but she won't even pick up the phone. If I can get her to answer me I'm sure I can fix things with her. Should I just keep calling or what?