Does anyone know why there would be a red bump on the tip of the opening of the penis?
It varies for each person. Generally the opening can be like a pair of lips puckering out, being flat, or it can be indented — and if it's the type that pokes out, then getting rubbed by or stuck to clothing can irritate it and make it puff up more than usual. Typically it's temporary.
How long have you been retracted? At first my lips puckered out for three or four weeks before not inflaming (and losing their boyish redness).
My lips usually get swollen after unavoidable constant rubbing all day being retracted. Seems normal if it went down in a few.