When to go to the hospital?

I'm 26 weeks pregnant, and still have to do a lot of lifting at work. This morning I was having a lot of Braxton Hicks contractions while working. At the end of the day I felt like I had a wicked bad menstrual cramp that made it hard to lift my leg to get into the car, but I felt better when I sat down for a while. Did I just strain something or should I be concerned?