Not sure about the back pain, but the fishy odor sounds like bacterial vaginosis (BV). That's the main symptom: a fishy odor. It usually also makes your CM more sticky than normal. I would suggest you go and see your doctor or obgyn. testing for BV is quick and easy, and if that's what you have, they'll just give you a round of antibiotics. Good luck to you!
Sounds like a vaginal infection. I would see my doctor I if I were you.