Pretty Sure She Came...Just Didn't Think It Was Possible

Ok, hi everyone. Right now, I am with the only girlfriend I have ever had. We have been together about 5 months and I am also her first boyfriend. We haven't had sex because she is determined to wait until marriage and I have not fingered her or performed oral sex on her. This has happened several times and goes as follows...
Basically, she is on top of me and we are making out and i am squeezing the very lower part of her ass pretty close to where her vagina is and we move back and forth a good deal doing this. The first time we did this, I had my pants on, and I am pretty sure she orgasmed because I heard her moaning for like 10 seconds and then a pleasurable scream that she says she has no idea where it came from. I know this question will come up but no she was not faking. She had never orgasmed before because she mentioned never masturbating and we weren't even expecting that to happen so she did not fake it just to impress me. The trembling of her entire body and the sounds were proof enough for me. The only thing I could think of was that her clit was rubbing on my pants but would that be enough or could it have been the lower ass squeezing that did it? Just curious, thanks in advance.