Please describe what you saw. If it really was mating you should see the female berried (carrying eggs) under her swimlets. If you check out some of the berried shrimp postings you may see pictures of what this looks like.
I highly recommend researching this website and forum postings as this topic has been discussed many times in the last few years. As a rule researching a topic should always be the first thing to do when a question comes to mind.