@pinkgin1
Owner said she was wormed and jabbed 5 week ago
As the first jabs are supposed to be at 9 weeks and the second set at 3 months, that would make her slightly over 4 months even if she got them done at the very earliest opportunity - which would make it about Christmas Day for the first vaccination.
Second jab five weeks ago, around January 22
Count back another 4 weeks for the first course - is she absolutely sure about that? She got her vaccinated Christmas Eve/Day?
I think that she is either older and thinner because she's been growing kitten(s) or she hasn't been vaccinated - or both. I wouldn't be surprised if she's a thin 5.5 to 6 month old as she's clearly not a baby, she's a young cat.
Whatever is the case with her, you'll be taking her to the vet, which will give the best chances for her and any kittens involved.
Feline pregnancy is about 63 days (around 9 weeks), give or take. If she's nearer six months, that's the perfect timescale for it all to be happening.
I think a call to the vet to explain you think she might be pregnant on Monday should get you seen quickly. It doesn't really look like she's just a bit porky (she's so thin otherwise) or has a bit of wind from changing her food - to me, at any rate.
She does look lovely - does she have a name yet?
Jezebel