Is my foster cat really pregnant? She's acting like she's in heat?

I'm fostering a cat from the local shelter which I work at. They sent her home with me saying that she was possibly pregnant and had an URI. They said she was very early along and likely would just be spayed once healthy instead of giving birth.

She does have a big belly, visibly LOOKS pregnant, but I cannot palpate anything and she has no other symptoms of pregnancy.

She does, on the other hand, act like a cat in heat - she is very VERY vocal, yowling all the time especially if she hears my cat meow(neutered male). She rolls around a lot, and is very affectionate. And the most pressing sign is that when touched near the back end, especially on back of back legs she sticks her butt in the air, starts kneading on all four, and kind of chirps.

If it wasn't for the possibly pregnant label and the large belly I'd say without a doubt she was in heat, but that confuses things. I admit that though I work at a shelter I'm not up to date of cat pregnancy/in heat behavior. She's quite gassy, could that cause the large belly? But I've never seen her without the larger than average belly, and she is not fat anywhere.

Would a pregnant cat act so much like an in-heat one? or would you agree that she is not pregnant?