is it possible ? 18 weeks

is it possible to feel baby pushing his/her butt(or something) out? if I lay down on my back for a few minutes, I'll get this small hard spot in my pelvic/lower ab area. if I touch it a few times it goes away. I know later inpregnancy you can feel or even see feet being pushed out but I feel like at 18 weeks they still have to much room to be able to push out far enough to feel. also I'm very small, only weigh 100lbs and don't have much if any of a belly.