I started getting a pain like you described this week! I'm almost 36 weeks. I think it's what stacey said since it's worse when I've been vertical for a long time - i.e., I don't notice it in the morning or if I lay back for a while but do after teaching on my feet all day.
is it in your cervix or vagina area? if its more in your vag area its most prob whats called lightening, and thats where babies head is rubbing on your cervix causing electric shock like sensations, they can also run down both side or just one side of your legs as well, they can be quite debilitating, especially while out as they can nearly double you over.... fun :)
Not sure what this means, just wanted to offer some support. This is my first pregnancy and I've had those pains since 36 weeks too. 38 weeks now and still having them and still pregnant. I hope someone can answer your question for you...