Fundal height measuring 4 weeks ahead but u/s shows different

FirstTimePregnancy

I saw the OB today and when doing the fundal height, I am measuring at 37 weeks rather than 33. I had previously been measuring 2 weeks ahead and now 4! The OB wanted to do an u/s to see just how big the baby was and according to the u/s tech, she is currently around 4lbs. 9oz. and right on track for 33 weeks. She also said my fluid levels looked good (not too much), so I have no idea why fundal height is measuring 4 weeks ahead. Don't get me wrong, I'm glad that everything looks OK...just a very confused FTM. Anyone else have this experience??? TIA!