Help with pains?

Can someone help me as to why I'm so uncomfy, last week I had a 28wk scan where she said he was head down, my belly is growing lots and I feel as if it's dropped a little, I'm due in 11 weeks and have a constant back pain which is currently in my lower back today, whereas it is usually in the middle right same as my rib at the front, I assumed it was him pushing on my rib making it sore, but my right leg feels so heavy and painful even if I only walk 10 steps it feels as if I've ran 10 flights of stairs using one leg, anyone have any reasons for these?