First of all, congratulations!!! What an exciting time for you. Secondly, yes, I believe you are over-reacting. From what I know about ectopic pregnancies (my best friend had one before she had three children), they usually occur before you even suspect that you're pregnant. The pain you are experiencing could be implantation of the fertilized egg in your uterus. I understand that there can be some pain associated with that. However, I'm no doctor, so you need to consult one if your pain continues.
I never had a miscarriage, and I have two grown children. There are many, many of my friends who never had one, either. You need to relax and get involved with something that will take your mind off of what you fear. (I should think running after a toddler would that care of that!) How about learning a new skill such as crocheting or scrapbooking or even getting involved with Facebook? Anything that will challenge your mind and keep you occupied.
I hope these suggestions help a little.