Lower Back Spasms

Ginna
I am in my third trimester and have started experiencing these strange lower back
Spasms which feels as if my back is throbbing or pulsating! At that time I also feel pressure in my chest between my breast bone! These spasms causes me to cry out in pain but it does not last long!  
Please tell me if any of you are experiencing any thing similar.  I will definitely talk to my doctor about it during my next visit because it scared me!