Dating me just for sex?

We're both seniors in high school and we dated for a little over 3 months. A few weeks ago, we had sex for the first time as a couple (we weren't virgins). Now I don't like or want to brag, but I have a rather large penis, 9 1/2 inches long and almost 6 inches around. Not to be vulgar be she came 3 or 4 times on it before I came. Now we usually hung out as much as the normal teen couple, but after our first time she literally came over twice a day for almost 2 weeks wanting to have sex. I've worried about it for a long time, a girl dating me sheerly for my size, and I got the vibe that's what she was doing. I confronted her about it and she denied it, but later one of her friends told me she liked me just for my size and I believed her. I broke up with her but she still messages me apologizing and saying it was all a misunderstanding but I'm not buying it. How do I know if she's telling the truth?