NEED YOUR HELP LADIES 😬

So I went for my first ultra sound today and am at 6 weeks 3 days. Everything looked good and then my doctor called to tell me the heart beat was at 115 which was good and that everything looks how it should. But that I have a small clot behind the placenta and she wants me to come in within the week to look at it again to make sure it goes away or at least doesn't get bigger. I'm so nervous because I've had a mc before and I just hope it wasn't for the same reason .. I was very young at the time and didn't see a doctor at all for that pregnancy. Has this happened to anyone else before ??