Spotty rectal bleeding (pink) after a constipated bowel movement yesterday. just smudges on underwear. no pain. should i be concerned? what to do?

Reduce constipation: most importantly, as it's often a sign of low fiber or dehydration that puts strain when you are defecating (pooping). So, you don't want to create hemorrhoids internally that are like vericose veins, and can burst and cause bleeding, even if you don't have pain. You create pressure when increasing or holding your breath to push out the feces. So, drink more water, and increase fiber thru veges.

Yes: Take more fluids, prunes, stool softeners for now, and see Md for stool exam in a week or so, to be sure no occult continued bleeding. May have been a one time item, BUT be sure.