What Causes Pain In Lower Back And Hips?

Hi , i am a 32 year old female, i reecently underwent reconsrtuctuction surgery in my vagina due to complicated bartholins gland removal surgery that led to complications. i also suffer from psoriotic arthritis. ive been on various meds for it ie: methotrexate, etodolac, amitryptylene etc. My problem at the moment is i have severe pain in my lower back, down onto the back of my hips and also half way up my spine ?? i have also noticed in the last few months something unusual which i can only describe as an indent on my left side, wear the lower back jois to the top of ur bum cheek, its on the same loevel as ur coxiics bone only to the left of this, its like theres a large dent that goes inward, its not on the other side and its very painful in this area. Any ideas what this could be ? i am very worried about it ? thanks in advance for any info u can give me on it ?