Spaying shouldn't cause really bad smells from (her vagina?) My dog was spayed last year and smells fine. If that is where the bad smell is coming from, it could mean she has an infection, either urinary, or even possibly what is called a "stump pyometra"
Does she have any discharge or slight spotting/outright bleeding....from her vulva?
Or it just may be possible this "rear-end" bad smell has nothing to do with her "lady-bits", and is from her anal glands!
I agree with Misfits4Me.....give as much information as you can, no matter what private doggy-bits you are talking about!
Welcome...It depends on what has become smelly and where are the infections located....What does "Smells bad from behind mean" ? More information would be helpful.....Thanks