Measuring behind and now ahead

So at 7 and 8 weeks I was measuring a week behind. At my anatomy scan they told me I was measuring a few days behind. Now my dr told me at my 24 week check up I was measuring two weeks ahead. That was just the normal measuring tape checking he did. I will have another ultrasound in 4 weeks. Has this ever happen to anyone? What could be the reason I’m measuring ahead? Sorry I’m a first time mom and learning every day. I just wanna make sure my little nugget is okay!! Thank you in advance