Talk to your GP as soon as possible.

If you can't get your GP to intercede, or if it would take too long, bring someone who will act as your advocate and doesn't mind being persistant, or a complete pain in the arse if that's required, and if the pain doesn't diminish or if it gets worse, go back. You should not have been sent home with that much pain.

I once drove my mom to an emergency room with the same type of pain. The doctor there sent her home and told her the pain was likely related to menopause. Well, it got worse and she called her GP who ordered her back. It turned out she had diverticulitis and wound up having a chunk of intestine removed. I believe her GP had a few words with the idiot who sent her home. Even if your pain isn't caused by anything life-threatening, you shouldn't have been dismissed as you were.