Google pelvic girdle pain. I get the same thing. It's horrible. I'm at 17 weeks and have it almost all day. It's worse for me when walking. My OB said its from the pelvis stretching.
I believe this happened with my 1st pregnancy too. My son is 4 so it was 5 years ago so this pregnancy is like starting all over LOL
I experience this randomly and one of my friends who has 2 kids said its normal.