8 weeks post pardum, rectal bleeding, constant lower back pa

Hi... I am 8 weeks post op from a c - section. I have had lower back pain and nausea for the past 4 days along with frequent urination and discomfort . I am on the 2nd week of my birth control pills (orthotrycyclin low) and now I am having rectal bleeding (about a half dollar size in my underwear) When I was 27 weeks pregnant I had a severe UTI and kidney stones.
Can you have rectal bleeding from kidney stones? or could this be a sign of something more serious?