Just asked. Took 4 month old sons temp with rectal thermometer. There was some bleeding. He has not been constipated. Dr visit or what should I do?

See doctor. Hopefully you used vasiline to insert. Protect infant from throwing buttocks against table as this could cause trauma. Need to also see physician to check cause of fever.
Wait and watch. The rectal thermometer might have caused slight bleeding if it was not lubricated or there is breaking of skin at the anal opening .The bleed may stop of it own and then nothing to worry about it .Wait and watch if any more bleeding then get the baby checked up by your doctor to sort out the cause of bleeding and get the treatment .