What Causes Stomach Movements With No Periods?

I have a 1 year old son and over the past 24 hours I have been experiencing feelings in my stomache just like when a baby is moving around & kickin, and over the past few months I have put on about 6lb, but I have taken a test & it was negative, and I use the depo injection as contraception & have never taken it late & I no longer have periods. Could it just be gas as some people say or could it be something more serious like a tumor