Once had a older gelding who thought he was a stud and mounted all the girls. My girl, had enough so she kicked him hard "you know where" poor guy was HUGE, vet had to put him in a sling (yes, a sling... ) and the said sling had to packed with ice to relieve pressure and swelling (poor guy).
No long term issues. If he's kept with other horses, he may have been kicked, in which case it would be good to get a vet to look at it. That's the only time I've a large amount f swelling... But could have been irritated if you just cleaned it. Did you use soap? What kind? Did ya make sure rinse rinse rinse? Cold, plain water may help.