Pregnant and experiencing shooting pain in leg?

hiya, im 20wks pregnant unfortunately my car is off the road at present, until such times I can afford to have it repaired, so I did alot of walking today which is good for me as Im so unfit! A few weeks ago we took a 2.5mile walk round a lake and I experienced the same thing then. Ok im fine whilst Im walking although my legs ache, when I sit down, my right ock seems to kind of seize up, and when I go to stand I find I cannot put any weight on my right leg as there is a shooting pain going from my bum down my thigh at the back, I cant really pinpoint exactly where the pain comes from I just know its really uncomfortable and so I need to limp around, it does feel like something has seized up, im not sure if walking it out would help, as Id need to go through some pain first! I was thinking sciatica but im unsure, also I had SPD during my first pregnancy but it doesnt really feel like that either. When I had it a couple of weeks ago, it just went away itself overnight Any suggestions? Thanks