I have noticed 2 light bruise spots on my back that have not went away they are on the top center where sometimes I get a muscle stinger at at times they itch in the spot not raised dark brown or sore what could this be I do have pics but don’t know how to post... is this how slim cancer presents?

Best is to see a doctor about it as we can not diagnos this in any way also you need to rule out if it was caused by a bug bite as from a spider. You could lose alot by waiting.
See a doctor.