Why can't I say "我的腰不好” My teacher said it wasn't a good thing to say - why? For example, my back really hurt - is this not what I should have said?

How do I express that my lower back hurts?
腰 or 腰子 is slang for kidney. In traditional Chinese medicine theory, kidney is responsible for producing and storing sperms. Thus 腰不好 becomes a slang implying weakness in bed.

It takes a cheeky friend to twist 腰不好 towards sex; In a lot of situations saying 腰不好 is fine. If you are unsure, it is always OK to say 腰疼 or 腰痛.