Ooh - I feel for you Rose! I'm the same - I had heaps of them when I was younger (and yes from sex) It's often if you don't go to the toilet before and after the deed that can bring them on....I haven't had one for years due to being diligent with it!!
They are sooo horrible and I would go and get antibiotics straight away as things like ural didn't do anything for me.
I've had one kidney infection from it and had to spend about 3 or 4 days in hospital with iv anitibiotics! FUN!
The antibiotics for me work withing a couple of hours so hope your feeling heaps of better now!