Hi Cas Blob
At this stage in your pregnancy your body is making more blood vessels and more blood. In most cases they keep up with each other but quite often your body makes blood vessels quicker than it makes blood. The down side to this is that your blood has to spread further to fill these extra blood vessels. This can and often does lower your blood pressure. It would be worth getting it checked.

Oh this can also work in reverse. Not making blood vessels quick enough can increase your blood pressure.