I doubt it is pregnant. They don't readily breed in most aquariums, unless you are specifically trying to breed them.
A few questions to diagnose the problem:
Does the fish look ok other than bloating?
Is the spine curved at all in comparison to the other fish?
Do the scales look like they are raised at all, or the eyes popping out at all?
Any changes in behavior in comparison to the other fish?
What are your water parameters? (nitrate, nitrite, ammonia, temp)
How long has the tank been established (if less than a couple of months, did you cycle it, and how?)