Pains?

Sasha • First pregnancy! Due on the 4th of December with a baby girl👼🏻
I'm 35 + 4. I recently noticed a pain on the back of my thigh/just above the back of my knee to the left side. It's about 7cm long, and only hurts when I try to put my leg straight or touch it (it feels like a bruise, but more tender and much deeper!) there's no mark though, just skin. 
I got my mum to look today and she said it is quite a bit more swollen there than on my other leg. 
Does anyone know what this is/could be?
I'm also still working full time on my feed 24/7 if that helps.
😩😩