Our son did/does this too. He has had four episodes: the first when he was 3, the next two when he was 4 and the most recent, now that he is 5. All episodes occured after our son had some form of injury (the first two he ran into other children in the course of playing tag and the last one he hurt his finger)and he was crying. Lucily for all but the last incident, we were holding him to comfort him so when he passed out he did not fall. The last incident we were not so lucky. I guess since the first 3 came relatively close in time and we had no incident for close to a year, we did not expect it to occur.
The first time it happened, my husband and I thought he was choking because his lips turned blue. Our pedi at the time said not to worry. After the second episode, we had a new pedi who said he absolutely should be looked at to make sure he is okay. After the third incident, we felt confident it was breath holding and that he was okay. Just something to keep in mind- you need to let all caregivers know about this. Our son had his third incident at pre-school. Luckily, they were aware of the situation and were able to handle it just fine. Also, after the first incident, we started telling our son to breath when he was crying. We tried to make him cognizant of his breathing.
My favorite thing about this is when you read about it (try googling "episodic apnea"), you are told not to react to it because it can become a learned behavior. I find it funny to think that someone would not react to their child's lips turning blue, eyes rolling back in their head and falling to the ground!
Good luck!