2yr old boy had blue lips and went limp in my arms....

My 2 yr old was fine a few weeks ago and all of a sudden he comes running to me holding his bum and screaming. Then his lips went blue and he dropped in my arms. I was so scared, I thought he was going to die. My husband and other son thought that he might have swallowed a penny the night before, so I thought maybe it was stuck. We ran to the hospital and he was on my lap in our van...I had to keep trying to get him to stay awake. When we got there, he laid there like he was in a daze and not really knowing what was going on. They did a chest xray and found no penny but the doctor said that his bowels were all blocked up. They figured that he might have Vasovagal Syncope. Anyway, they gave me an enema liquid for him and sent us home. I've been terrified since worrying about it happening again and not knowing what to do. And not really knowing what exactly it is that he has. He always complains of a tummy ache and when he has to have a bowel movement, which he hasn't been doing alot, he screams and freaks out. Can someone help me figure this out???

Thanks,
Dawn