Hello girls. I'm 6 weeks pregnant. Yesterday, I had bleeding. I didn't…

Hello girls. I'm 6 weeks pregnant. Yesterday, I had bleeding. I didn't feel any pain but I called an ambulance. I waited for two hours for the ambulance. They checked my belly with ultrasound and said to come back on Tuesday. They told me if my stomach doesn't hurt and you feel good, you can go home. What do you think could have caused the bleeding? The doctors didn't explain anything properly.