Floss - i think your tum is fine as well. Me, at 16weeks, had put on about 2kg.
Everyone is different, and everyone carries differently. An interesting read in third trim section, there is a thread in there about how much weight ppls have put on, it was good to read the differences between people in weight gain.
At the end of day, i think as long as bub is growing fine, and you are feeling great, thats all thats important.
There is a good line that i read in a book, regarding peoples comments to you, - "remember its about them, not you" ie, their own expereiences. They think you are to big/small in compared to themselves when they where 'x' weeks preg.

As for pain, ditto to the others as well, just bits stretching. Everytime i go to obs, i tell him where my pains are, and he says yep, all normal. Sometimes i get as far as just starting to point, and he descibes the pain to me, and i go, yeah, thats it, and he says completey normal.