why is she growing so fast!!!

Tonight when I was getting my daughter ready for bed I put her in a 3-6 month sleeper and it was really snug and essentially to short so I put her in a 6-9 month and it fit her like it was made for her. After I dressed her I stood her up and she was putting weight on her legs (she's been doing that for a while) and I just looked at her smiley little face and it's so hard to believe just five months ago she was a little teeny tiny newborn 😭😭😭 I feel like just a couple weeks ago and I'm going to need her to stop growing up so fast!!