I HAVE HAD A SENSATION THAT THERE IS A WET

I HAVE HAD A SENSATION THAT THERE IS A WET SPOT JUST BELOW MY SHOULDER ON MY UPPER LEFT BACK FOR SEVERAL WEEKS. IT COMES ON LATE IN THE AFTERNOON, OFTEN WHEN I HAVE BEEN WALKING MY DOG. FOR A LONG TIME I THOUGHT SAP FROM A TREE HAD FALLEN ON ME, BUT OF COURSE THERE IS NOTHING THERE - NO PIMPLE OR ROUGH SPOT. MYSTERIOUS. Could it be anything serious?