Abnormal AFP results?

I am 18 and 1/2 weeks pregnant. I had my genetic testing and AFP tests ran at 16 weeks. I received a call today from my OB doctor who said that I needed to come in tomorrow because my AFP was abnormal and I needed to have my ultrasound early.....My husband and I are so nervous and scared...Please let me know if you have had this problem or what to expect at our visit tomorrow. Any advice will be helpful. Thank you.