Doctor wants another sonogram?

Well yesterday i got a letter in the mail from my OBGYN saying they have scheduled me for another ultrasound (we just had the big one to find out the of the baby) they didn't give a reason but my OBGYN doesn't do anymore than what they have. Has anyone had anything like this happen? Obviously they found something they need to check again. I can't talk to anyone until Monday. It said in the letter they tried to call but didn't have my correct number. Any advice is appreciated!!