Last weekend I couldn't stay hard with my girlfriend, should I worry?

So I am 21 years old, started dating my current girlfriend about a month ago, she is literally the most beautiful girl I've ever seen, has an incredible body and an INCREDIBLE sex drive (usually would be amazing in my eyes). I usually am a pretty sexually able guy, in my past I've never had problems staying hard, and didn't at first with my current girlfriend. She goes to college a little over an hour away form me, so we have to wait for the weekends to see each other. First two weekends I went to visit her, I had no problem performing, we smoked a little weed and later on in the night we had a great time together. I stayed hard, everything went great both times. The weekend after that, I went home to celebrate Thanksgiving weekend at her house, met her family etc. Of all the places to get shy, you'd think that would be it right? Nope, I still was hard as a flagpole with her. So we had sex a few times that weekend, no problems. Now last week began, I had no classes on Wednesday so I went to spend Tuesday night with her, while I was there she revealed that she thought she might be missing her period, and might be pregnant. This got me really nervous, and later that night I had trouble staying hard, but I was able to get her off a few times by performing oral on her and using my fingers as well. I thought it was just the stress, so the rest of the week I was living in fear that I might have gotten her pregnant. That weekend (the most recent past weekend) I stayed Friday/Saturday nights with her, Friday night was a normal night, went to smoke with her friends but I think I might have smoked a tad too much weed because I was really high. Later that night I was unable to stay hard, even though she told me she had taken a test earlier and she ended up not being pregnant. Then Saturday night, she had a little too much to drink and I ended up tending to her / helping her try not to throw up.

I really like this girl, and know there is absolutely no issue in being attracted to her physically, so is this just a mental thing because of the pregnancy scare? should I just relax and try not to smoke so much next time? or should I see a doctor about it?

Points to the best answer, etc.