Kitten LOSING Weight

This morning was my kitten's third and final vaccine initials appointment at the vet. She will be 17 weeks old on Thursday. On January 9th she weighed 5.3 pounds and today (4.5 weeks later) she was down to 5.12 pounds (!). Her body score was 5 at this appointment, which means she's thin but not emaciated. She eats a 3oz. can of Nulo all-life-stages wet food daily, plus free-feeds Nulo all-life-stages kibble (perhaps a 1/2 cup total but it's hard to tell since she kinda throws it everywhere). I'm quite worried that a kitten has not only not gained, but has actually lost weight at this stage of development. She did have a bit of a bout of mushy poops for a few weeks last month, which has now resolved and never did progress to outright watery, and a fecal PCR at the time showed no issues. She's very lively and the vet was "not worried yet." Has anyone seen this kind of thing before?

UPDATE: I reached out to the breeder, because I was absolutely sick with worry and wanted to know if she'd seen anything like this before. She pointed out that the kitten weighed 2.1 pounds on December 12th (she also sent me the records for that visit) - which means she's gained 1.6 pounds a month for the past two months. I understand that that's ideal. The breeder also relayed that three of my kitten's littermates were just taken to the vet and weighed between 4.7 and 5.6 pounds. So my kitten's weight is right in line with her siblings. The only conclusion is that my vet mis-weighed her at the January 9th appointment and scared the sh!t out of me for no reason whatsoever. The kitten is exactly 4 months old and weighs 5.4 pounds, which any vet should have known is not worrisome.

THANK YOU so much to everyone who responded.