First ultrasound!

I had my first ultrasound yesterday & went in thinking I was 7 weeks (based off my last period). The doctor said the baby was too small to be 7 weeks he thinks 5 weeks. He said this is normal because I just came off deppo 6 months ago & my ovulation could have been late. Good news is he saw a heartbeat and he wasn't concerned about anything... I'm still nervous though! Anyone else have this happen?