help!!! I'm pregnant and don't know who the father is.

So I've been single for almost a year now, and last month I was seeing 2 guys, one of which I'm pretty attracted to and the other who was just a fling I've had on and off since September. I had sex with both last month, with one of them (who I'm gonna call Jack) I had unprotected sex several times last month, I can say we are pretty steady and we have sex at least once a week. But the day I remember doing it and could've possibly been the day I got pregnant was Oct. 27. He didn't pull out or anything but he's always thought he was infertile because he never really produces a big amount of semen like other males. And with the other one (who I'm gonna call Pete) I only did it on Oct.18/19 on a short trip to the beach. He did pull out but for some reason there is this thing running through my mind telling me he is the father. I don't know if it's because I'm unsure and I only told Jack that he is the father. My last period started Oct. 14th. I'm not very regular. I'm 6 weeks pregnant now. Who do you think is more likely to be the father????

 

Like I said there's a little bug saying it's Pete but we only did it like twice on the 18 and 19th and I haven't done it since then, while with Jack it's been more times but he never really produces a lot of semen which is weird. I'm so worried on making a wrong assumption. Please help me figure this out!!