Hey,
it could be one or both!.. My sis who is having a boy this time around (2nd pregnancy) she said she felt his sitting right down on her pelvic bone as soon as 13-14 weeks... But her doc also said that it was normal cause it was her second time around and all the muscles and stuff are all stretched and such!....
On the other hand it could also be, that you are just constipated.. The one thing I know for sure is if you are taking marterna.( the vitamins) they make you constipated.. (or at least they do for me!) and my doc has told me that many complain about the same problem...
Talk to your doc, and see if he/she would sugest you taking a fiber pill or something.. (my doc told me to take medamucil once a week to counter re-act what the maturna does!)
anyway hope that helps a little..
Best of luck with the future baby!
Lor~