I just had my 32 week appointment and my belly measured 30. My midwife wasn't concerned. She said 2 weeks small or big is fine.
It's probably nothing. Fundal height is a pretty inaccurate way to measure baby, so they're doing an ultrasound as a precaution.
I had one at 38 weeks with my first because I was measuring behind as well. At the ultrasound they said quite the opposite- he was going to be a large baby. He was born at 6.5lbs. They don't have great ways of measuring size at weight at the end of pregnancy.