I just got a call from Dr. B's office saying I need to come in for a snapshot ultrasound. Unfortunately, I am filling in for someone at work all week and it will be almost impossible for me to get there (I live 1 1/2 hours from their woodbury office)

Can anyone tell me why I would need to get this done? I am not due for AF until the 10th.

Will I be messing up my possible cycle start if I can't go?
*******UPDATE*********
I emailed Dr. B and asked him about this and he said that in order to do a luteal phase start on me for the June cycle, he will have to see me in this luteal phase or I will loose the opportunity to cycle in June. When he sat down and went over the dates he realized that if I wait for my next luteal phase we will miss June. He needs to see if I am in my luteal phase tomorrow (which I am pretty sure I am) and if so I would start lupron now. (yikes!) He also mentioned a ER the first week of June. Oh my word, this is all happening so fast. I am scared and excited all at the same time.