H.B.
I have had this pain when pregnant with my first two (although not with this one, maybe because he hasn't been head down?), and I don't think it's a pulled muscle, it's probably caused by the pubic symphysis, where the pubic bone meets and has a seam, moving and even separating a bit. I know it hurt worst when I was on one leg, like putting on pants or going up or down stairs, like you said. Talk to your ob about it, although I don't think there's much they can do, but they may want to rule out any other problems. Some friends of mine have suffered this pain to an almost debilitating degree, and seeing a chiropractor can help. If you google pubic symphysis or pelvic pain in pregnancy you can get some helpful information.