Weight loss five weeks pregnant?

I found out I was pregnant at 4 weeks and 3 days. Last I weighed myself was exactly one week ago at 185. Today at 5 weeks 1 day I'm 177. Is it normal to lose weight this fast early in pregnancy? I don't see my doctor for another 3 weeks. Also, I started a running regime about 4 days before I found out in case that has anything to do with it, but I don't think 4 days of running will cause me to lose 8lbs in a week. Have not been sick just (tmi) pooping constantly.