i also felt the same kind of thing... i described it almost like a bubble bursting up against my stomach on the inside ha!! everyone told me i was too early to feel it but when i went for my next scan i seen him kick out and i felt the same feeling again. its a lot more defined now but yeah it sounds about right for u :-)
I was about 50 lbs over weight when I got pregnant and I've lost 12 lbs but im not thin but I think it just might be the baby! I had an ultrasound at 10 weeks and my baby was like a little bunny rabbit just kicking away so I know when the baby gets bigger the kicking is not going to be much fun!
That's what it felt like when I first felt my babies, like someone tapping or thumping me from the inside. If you are thin you can feel it earlier.