I have been suffering with back pain (middle of my back, both sides) about an hour after eating. My abdomen is slightly bloated too and I have slight nausea. The majority of the pain seems to subside after a bowel movement but i'm constipated. When I do have a bowel movement its full of mucous. Have also had problems with irregular vaginal bleeding for more than year despite taking the pill.
You should get a check done by your gastroenterologist for this and you might need to do a ultrasound scan too.