My neutured, male Basenji does this sometimes when he wakes from a nap and the pattern is always the same. There are two very hard acorn size lumps on either side of his penis, closer to the base but not as far back as the scrotum area.
Yes, that would be the description of both of my neutered males. Usually after sleeping or relaxing, and it disappears quickly. Maybe erotic dreams at work? Pretty harmless, in any case. 🙂