My mum has a red blotchy rash all over her body, especially her legs. some are small and some are.big covering her elbows. what is it?

Red blotchy rash: It could be almost anything. I would have to know how long she's had it; what medication she may be taking; how old is she; and does it itch? From your description it could be a rash from a medication she has be taking; she may have a condition called psoriasis (happens often on the elbows); and without a photo there is no way of telling. A dermatologist would be your best bet for her.

Uncertain: This rash would likely need to be examined and more history obtained to be able to provide you with useful input about the diagnosis or possible treatment.