My heart beat rises when I lay on my side.

I'm 25 weeks (tomorrow)
My doctor told me not to lay on my right side because it slows bloodflow to myself & my baby. & that laying on my left side was the best way for us to recieve blood. But whenver I lay on either side my heart starts racing really fast & I lose my breath. So I usually just spend the night sleeping on my back. I'm not sure if it's something to worry about or not, but I figured someone would know why it happens or what to do.
Thanks guys !

If its right after you lay down it could just be from you laying down. The extra weight and such from the baby and lung restriction could be causing it.