When you take your BP lying down, do you wait a while and rest?
I take mine sometimes right after I wake up in the morning, before getting out of bed, and I get readings of 110/55. My doctor thinks that's fine.
If you worry, ask your doctor next time you see him/her.
I guess its just super weird to me to see such low numbers when on meds I was pulling #'s 160/130's after delivery you know? Its like Im shocked! But I guess in a good way!
Taking BP laying down is a bit tricky too, because you have to have your arm at heart level, not just resting on top of or under your body. That may skew the numbers.