Should I be concerned if my blood pressure drops in pregnancy?

Not necessarily. If it remains in the normal range & you are not having symptoms this sounds ok but please talk to your ob.
No. We divide pregnancy into three "trimesters". The first trimester goes up to thirteen weeks, the second trimester is 14-26 weeks. During this second trimester the blood pressure will normally drop a little. You may be extra sensitive to this change and feel light-headed, tired or dizzy, drinking extra fluids, extra rest, and wearing support hose may all help. Make sure you are not anemic.