I'm 20 weeks pregnant with fracternal twins and my 4th pregnancy. I have placenta previa and an 40 yrs old. Every time I walk my uterus hardens and does not let up till I lie down. If I cook a meal I have a hard uterus the entire time. I also have a tingling in my vagina area afterwards when I lie down.
My question is if this is a concern.
I'm not seeing my doctor for another 2 weeks.

Not necessarily a concern, especially since it lets up when you are not on your feet. Mothers of multiples tend to have more experiences with uterine hardening, especially when they are up and about.

Since it is enough to cause you concern, you can always give your doctor a call. There are times when a doctor might decide to have you use medication to help calm your uterus.