I've also noticed however you've had these concerns in the past. Before you two engage in more sexually activities (an old post from last year put you at 17. I would assume you two are under 20) that you and her look more into what it takes to get pregnant.
If she's a virgin, why would you think she's pregnant? Symptoms don't mean anything-- at all. You also didn't mention her age. Puberty, hormonal imbalance, age, menopause, etc can all cause symptoms. Frequent urination can be caused from intake in fluids, UTI, etc etc. People in general can notice more smells. I'm not currently pregnant and can smell things no one else in my house can. "Thinking" you have gained weight is very different than actually gaining weight. If you guys are concerned, why not take a pregnancy test? If she's pregnant it will show accurately. That or you can wait and see if her period comes. If it does, she's not pregnant. You need to tell her however to NOT stress about it as stress is one that CAN cause a late/delayed cycle.