It can also be constipation.
I wouldn't be concerned unless they're accompanied by leaking of a significant amount of fluid or if they become constant and close together.
No you shouldn't be concerned, it's very normal to get them early. Everybody's body is different so you're just experiencing them early.
Should I be concerned at 15 weeks?
That's what it sounds like, Braxton Hicks contractions. Some people feel them very early, they technically start at the beginning of pregnancy. Must don't feel them till later. I didn't feel them until two weeks ago after a car accident.
I'm pretty sure they are contractions!! Usually your stomach tightens during contractions.