[Commiseration]

It's good that you're concerned for your girlfriend! But I don't think you should be stressing over this quite that much.

If she's having regular periods, it's pretty unlikely that she's pregnant.
Are you concerned because you didn't use protection at one point or something? If she's on birth control it's even more unlikely she'll be pregnant. And if your method of protection failed and she didn't know about it (you said she doesn't realize the possibility of pregnancy), you should be honest with her. Your best option is to talk to her about this.

I don't know where you are from, but it's very common to find pregnancy tests in drug stores, supermarkets, and other stores. You don't have to be any certain age to buy them. Talk to your girlfriend first though. She may find it weird if you just suddenly give her a test to take. Also, I wouldn't mention her getting "fatter" in case she's sensitive about her weight. She may have just gained a little bit. Again, she's having regular periods so I doubt she's pregnant.

Mostly though, just talk to her. It's all about communication!

Take care!