Why. is it that when i have a bowel movements sometimes i get back pain?

Depends: May be linked with a history of chronic constipation. Can be linked to positions you have to take to assist with a bowel movement. Can simply be back problems that are magnified when sitting on the toilet for some time. Talk to your doc about it and see what he/she says.

Infl. pelvic joints: Possibly this pain is due to mildly inflamed SI joints or inter vertebral facet joints that become painful on increased intra abdominal pressure e.g bowel movements. If indeed you are pregnant at age 55 then this is even more likely.