I had the exact same thing happen with my DD who just turned 3 and went into the big kids room. The first few weeks she was fine then we had hysterics for a couple of weeks and this week she was fine, still a little clingy but the teachers come over and occupy her a little to let me leave. She also stops crying minutes after I walk out the door.

It will pass, but I know how aweful you feel at the moment. Just thinking about it now upsets me.