I.am.at.my.wits.end.

I have a wonderful little dayhome going. I have been opened since October and have had nothing but positive feedback from parents about how happy their kids are with me and how they wish they had found me sooner (2 families left other dayhomes to come to me). I have a small group of fabulous kids...my biggest problem??? MY son.
He's 21 months old and going trough an absolutely horrid phase that start about 3-4 week ago. ONLY during daycare hours he has become incredibly aggressive: hitting, pulling hair, biting, screaming and even head butting.
For the record, I am being very consistent with teaching being gentle, emphasizing on the feelings of the "victim", sitting on the stairs for 1 minute and redirecting to a new activity. But it's only a matter of minutes before he strikes again. If I'm not RIGHT there (I left his side today to go fix a skipping CD), he WILL hurt someone. He bit a 19month old dcb and while I was fussing over and icing his bite mark, he ran over to a dcg dancing to music and pulled her hair and wouldn't let go.
I'm just at my wits end!!!!!!!!!!!! I feel like such a horrible Mom, because I DON'T KNOW WHAT I'M DOING WRONG. I have a 7 yrs old girl who I didn't have any issues with this. I have my dck's whom I'm able to manage very effectively any aggressive behaviour. Today after that last incident I described, I brought him upstairs away from the other kids, went into the bathroom and had a full out sobbing break down. It was just one of those weak moments where I felt so helpless...my poor husband on the phone was terrified lol.
Anyways, they are all sleeping now so I get to chill...hoping for a better afternoon. If you have any advice or possible explanation I would love to hear it...but this was more for venting purposes than anything. I'm sure it has something to do with "sharing Mommy" and getting more attention, but I'm just so frustrated and quite frankly saddened by the whole thing.