Why would a male have a swollen penis at the bottom of the shaft at the base?

Trauma,allergy,infec. Swelling at the base of the penis could be due to trauma to an erect penis due to masturbation or a direct blow. Non tenter swelling could be due to an allergic reaction. Redness and fever plus tenderness could be due to cellulitis, this would require antibiotic therapy. Suggest you see your md or do.