My 11 Year Old Son Woke Up Fine , We

My 11 year old son woke up fine , we chatted everything was normal. He got in the shower and as soon as he got wet, he suddenly had a terrible pain in the back of his head on the right side. he held the back wall of the shower and collapsed on the shower floor and felt nauseous. He regained strength but stayed on the floor in case he was sick. He got out of the shower, dried and dressed, the head ache subsided a little. He took children's Advil and felt much better within 1/2 an hour. He told me that this is about the 5th or 6th time this has happened in the last 8 months or so. What could be the problem?