I hope that you feel better after your appt...

I think that it's really a case of every baby being different. The baby could be laying with it's back to your belly button and therefore all the kicks and punches would be toward your back...or since you said the baby was not super active on the u/s it could mean that you just have a super gentle little one in there.

As stated above, the placenta placement also has a lot to do with it.
With my 2nd son he was so easy to feel both on the inside and on the outside- sometimes making my tummy really shake already- at about 15-16 wks and there was no mistaking that it was him throwing hard jabs. This time I figured I'd feel much of the same at about the same time...Not so much. I did feel flutters early but then I'd go some time with out feeling anything (and that made me worry!) and it's really been only in the last couple days that there have been the slightest little thumps felt from the outside- longer than it took with my others. I didn't figure it would take this long being that it's my 3rd!
Anyways, let us know how your appt goes...I'm sure that you will get to see your little one moving happily and freely in there just as content as can be!