Had Sex With Clothes On. Partner Pregnant. How Could This Happen?

My girlfriend and I were dry humping. She usually wear shorts and underwear while I m wearing jeans and underwear. I eventually ejaculated and my jeans is wet and I don t know if her shorts got wet too. After that she missed her period for 3 months and we took a home pregnancy test and I was shocked when she told me the test was positive. Now I am wondering if it is possible that sperm could travel through my underwear and jeans after I ejaculated although i know it is wet going inside her? We took 2 hpts yesterday which is 1 month after the positive result. We used 2 brands(1 is the same hpt we used which came out positive which I also believe it's false positive and the other is a different brand). The 2 turned out negative. Which would we believe? We're confused right now.3 negative and 1 positive with a very faint 2nd line. She hadn't got her period until now.