You didn't specify what your dosage is, but since it's your first cycle, I'll assume it's Clomid 50...? In which case the late U/S may just be to check whether or not your body will release the mature follicle on it's own, or whether you require a trigger. I seem to recall my first Clomid cycle they did something similar, although a bit earlier than CD21. I would definitely call and ask for clarification though. If you don't ovulate on your own, CD 21 is pretty much too late to trigger and it winds up being a wasted cycle.
Cristeen ~ Always remembering our warrior ~ Our is 3, how'd that happen?!?!
We welcomed another warrior in May 2012!!
2012 Decluttering challenge - 575/2012