Almost every night, my 3 year old son takes off his pants when he goes to bed,

does a leg exercise he's been doing since he was about 6 months (he calls it "Pilates," as in "I'm doing my Pilates") and gets an erection.

At some point, he complained that his stiff penis hurt and Mrs. told him to put a cold rag on it. Since then, almost every night, my pantless son gets up from his bed about 5 minutes after laying down, goes to the bathroon, and walks back explaining that he just needed to "get a cold rag for [his] penis."

Is this normal?