Spotting while pregnant

So this ismy 4th pregnancy. I ovulated nov 7th and knew I was pregnant 1 week after. Around what should of been 5 weeks I started spotting and it didnt stop for 8 days light pink I went to the er and they did hcg which was 241 the draw two more times it was 546 and then 1458 and I was 6weeks by the last hcg test so It was more then doubling but i was still spotting. So I'm curious could I be pregnant with twins I've never spotted during any other pregnancy and for some reason I keep thinking theres 2 in there they never found a source to the bleeding but I do have an ultrasound tomorrow morning.

Thanks