Thank you for submitting your question. First of all, it is not uncommon for women to not have menstrual cycles for a few months after stopping birth control. The body needs to regulate itself. This can last up to 3-4 months.
Since your pregnancy test is negative, the movement in your lower stomach is not from a fetus. In addition, fetal movement is usually felt at around 18-20 weeks. Even if you are pregnant, the fetus is too small to produce any discernable movement.
In most people, the sensation of "movement" in the abdomenal area is from gas. This makes the most sense because nothing else in the abdomen actually moves except for the intestine moving feces and gas through the bowels. If your sensation is more consistent with pressure and your abdomen is actually getting bigger, then an ultrasound should be done( to rule out fibroid for example).
I hope this is helpful to you