Hip Joint Pain

I'm 21 weeks and for over a month now I've had horrible stiffness and pain in my hip joints, in the back where they meet the base of my spine. It's worst at night for some reason, and sometimes it's so bad I have to crawl up the stairs to get to bed! I know all my ligaments are loosening and things can get out of alignment, but if I didn't know any better I would think this is arthritis. Is is possible for pregnancy to cause arthritis? Has anyone else experienced this? I've been stretching every day and taking Tylenol occasionally for pain, but nothing seems to help for very long.