Sudden constipation

So Tuesday I took my girl iris, 2 year domestic short hair, to the vet because I noticed she passed some blood in her stool. Other than bring over weight, which is being worked on (14 lbs), she appeared normal. Well the vet gave me albon? An antibiotic. The only way I could get her to take it was mixing it with wet food. Otherwise she'd just drool it out of her mouth.

Well since she's on a diet and is taking two doses a day she's only eating wet food. And so is her sister since I knew if I only swapped one that likely iris would steal kibble too. So they've both been swapped to only wet food.

This is when the issues started. I'm supposed to collect a stool sample from iris so I've been paying even more attention to her bms.. she hasn't had one since Tuesday. Neither of them have. I can tell they are peeing normally. But I've also noticed they aren't drinking as much.. should I be panicking?? Or is it just from going from a super dry diet to a super wet diet suddenly? I realize now I should have probably done it slowly but well... I was worried about her not getting the meds and over eating..