I think I had a miscarriage but I want to know the difference between a gestation sac after 3 week pregnancy or a uterus lining during a period.

See your dr. If its only been a little bit of time since you had bleeding that was either a miscarriage or a period, the doctor can check a test to see if you are still making pregnancy hormones (which you may be if it was a miscarriage). If you don't want to be pregnant, you should talk with your dr about birth control choices. If you do want to be pregnant, plan ahead, start prenatal vitamins, and be health.