Yep - totally agree with the other posters. Siatica (sp). See a physio hun (should be able to access one through the hossy where your going).
Has to do with how/where the baby is sitting too.
I had it for the last 6 weeks with DD and have had it since 6 weeks with this bubba. SOOOOOO painful but I find regular physio/massages and heat really help it. Physio told me also to avoid long walks, still walk just not the massive ones I was doing.
HTH x