If it is soon afer having your baby it is probably your uterus contracting back to normal size after growing for the baby.It could also be your internal organs moving back into place after being "squished" to make room for the baby.
I still felt "phantom movements" for months after having DD.
She is 7 mths now, and I still get the occasional strange feeling in my tummy. Not sure what it is, but I suppose after 9 months of pregnancy, and all sorts of weird things happening to your body, I guess its only natural that things will take a while to go back to normal lol
i actually started a simalar thread about 5 months after i had DS cause i too felt like i was pregnant and the baby was goin crazy in there... its just soo empty in there now i think the littlest things seem big... if you know what i mean...