I know what you mean - this was one of the first sensations and signs I had with my second pregnancy. I think it is due to the uterus being about the size of a grapefruit at 8 weeks and the increase in blood flow/volume around the body. Also, you may have slight cramping/contraction type feelings which can be worrying but I can only go by my experience and say that I went onto have a healthy baby girl when I was 43 years old.
I worried about every little twinge (and bleeding) and change my body went through right up until she was born.
I hope everything is ok with you and that you enjoy this exciting time......x