Was this an ultrasound measure or a belly measure?
I grow REALLY big placentas.
Typical placentas are 3-5lbs. Mine was over 20. Between 10lb baby, 20+ pound placenta, and amniotic fluid... I was carrying over 40 pounds in my uterus. I was a whale. Or a kangaroo. Or whatever. Freaking huge.
I also had a month long 'due date', that actually got schlepped around quite a lot. There are some golden times to 'see' gestational age, but outside of those there is a couple week margin of error. So if you don't know date of conception, you may be quite a bit further along than you thought.
While there are always "yikes" reasons to fret about, it's probably a common one:
A) big placentas, lots of fluid, baby chilling out in a wacky position that minute making you measure big, etc.) if it was external, or
B) if internal/ultrasound then most commonly you got your dates mixed up. SUPER common as many women still have a "period" (that isn't actually a period) their first month. Meaning you may have a whole month knocked off your sentence!!! (Okay, I hate being pregnant, so i do realize that might not be good news for everyone.)
So at 21 weeks, you measured 3 weeks 'ahead'... Give or take 2 weeks not in a golden zone may very well be you were actually 5 weeks ahead from that point. Or conversely, this measure may be off 2 weeks, and you're actually only 29 / still just 3 weeks off.
Go in for the US (or next one) before fretting.