If a female/any person is so worried or scared, can they're stomach move like a heart beat?

First off, you are WAY too young to be having sex. And too immature. I can tell by the questions that you are asking and the fact that you are under 18. It doesn't matter if all the other kids are (claiming that they're)doing it. Worrying about whether you are/aren't preggo is not fun at all and not something you should have to worry about while you're still in high school.

As for your question, the only surefire way to tell is by taking the pregnancy test. Smoking may hamper some sperm. But guys produce 300 million new sperm every single day. So the pot smoking will not prevent you from getting pregnant. Your boy sounds like he needs to learn a few things himself. I really think you need to get your butt over to a planned parenthood clinic to get on some birth control if you plan on continuing to have sex. Don't forget to get checked for stds either. If you are too embarrassed to do this; you are much too young for sex.
I don't mean to sound like I'm preaching here. I'm just telling you what I wish somebody would have told me. :)

Yes, you can feel your heart beat in your stomach sometimes. I can pretty often, and there's no way on earth I'm pregnant.

And I don't think weed kills sperm to the extent that it's effective as birth control.

How long ago did you have sex? The chances are very slim, stop worrying as you could delay your period from stress, that in turn will stress you more out. And by the way how does your boyfriend know his sperm cells are mostly dead, has he had them counted? I take it he is under 18, yeah smoking weed kills sperm cells, but long term, like over many years, definitely not in his case so. Always wear protection. STOP WORRYING, Things will work you

You could be pregnant because condom's are only 99. 9% safe and even if he smokes weed I don't think it kills the sperm because my ex used to smoke a lot and I got pregnant