[Harmony♥]

The chances are incredibly slim that she's pregnant. She's had consistent periods, monthly, each lasting the same amount of time. If she were pregnant, she'd only experience light periods that last a day or two before completely ending. Even then, it's not really a period because she would be pregnant.

Honestly, you're worrying over nothing. Her weight gain could be from loads of things, from bloating because of her period, to water retention to an allergic reaction to something. The fact that you've never had intercourse also shows that the chances of pregnancy are very slim.