scan 10 weeks and 4 days

i had a scan done at 10 weeks and 4 days due to cramps. at the time of the scan the lady said everything was fine but the next day i got a call from midwife saying they picked up on a little extra fluid behind the neck. they want me to go to the hospital to do some more screening. im a little unsure what to do as i didnt know this test should even be done this soon? how common is this at this many weeks and turns out to be nothing?