What Causes A Bump On The Skin Of The Penis?

i have a bump on my penis mid way up the shaft. on the skin(moves with the skin) it apeared i believe in just one day and has been the same now for 2 days going on 3. it is aproximatly 4mm. not perfectly round. no sirious discoloration maybe a little darker arund it and a little lighter on it but barely noticible. no pain. no oter simptoms