Please help!

My discharge is white and creamy. And I've always smelt good down there tbh, kinda sweet. But I started smelling like fish down there. It's really bad. It burns down there. I've been having lower back pain and cramps. I have sex with my boyfriend but neither of us have stds. I've got tested even after we started having sex. I do have the mirena iud. And I know that my body has been weird. I just got Bell's palsy and started taking meds for that. I'll go to the gynecologist but I'm just curious what it could be. I'm tired of it.