I have it too and my doctor said its from be being overly dehydrated and from being dehydrated it caueed me to pull muscles in my back
I get the same thing and it is horrible!!! Its your syatica (not sure if thats how you spell it) and it hurts me alot, from moving when laying to putting pressure on my foot when I walk!! Hope it gets better !
If it's getting to the point where you can't handle the pain then you should definitely go get checked........even tho lower back pain is very common is pregnancy.