The most likely case is that you're pregnant, but it's possible that you might have a fibroid tumor (which can grow to be the size of a baby within a few years) or an ovarian cyst.
Please check with your doctor to get this looked at. Both conditions are not serious, and can be treated, but you have more options the earlier you're seen. There are also other conditions that can cause these same symptoms, though rare, are very serious.
Please see a doctor and let us know how it turns out.
Take care.