What Causes Vaginal Spotting During The Sixth Week Of Pregnancy?

Hi I had a first ultrasound which put me at 6 weeks 3 days with yolk sac at 3.1mm diameter and 6.59 crl. Heart rate was 114bpm. Then I had another scan a few days later as i had abit of spotting and was worried as I'd had a miscarriage 2 months before and was very anxious. There was 5 days between scans. The last scan I did showed crl of 8.7mm which put me at 6 weeks 5 days? Thr heart rate was 133 bpm. It does not tell me how big the uterus measures nor the yolk sac measurement on the last scan? Is that normal?