Re: severe lower back/pelvic pain
If your back has been rules out as the cause, then you need to start investigating what else it could be. In the lower right side you have intestines, female reproductive organs, muscles, your hip joint....all sorts of stuff. With the history of constipation, you could start with a gastroenterologist and see if it's the intestines. At the same time, I'd see a gynecologist and have that route eliminated. You could be seeing the effects of a hernia or arthritis in the hip. It's all about eliminating them one at a time until everything has been checked.
But since you have other symptoms of constipation, start where the signs are leading you. But do see the GYN too as ovary issues can start with constipation too(and bloating). One by one and sooner or later you'll get your answer.