Questioning Buffy's tummy!

As I'm fairly new I'll run over the basics!!!
I've had Buffy for nearly two years but I have not yet registered her with a vet. Unfortunatley life in my mad house is so hectic that things get put on the back burner for a while and are oh so quickly not done. I am ashamed to say that the vet was one of those things.
Buffy is a very healthy cat, she can seem slightly small but she was the runt of her litter and I just feel in love with her. My friend has a ginger tom the same age and he is like a tiger in comparison! Is it usual that queens are smaller that toms?
Anyway, her tummy has increased in size and my concern is that she is pregnant. It is not planned but the idea is not awful and we will love the kittens every minute. My problem lies in the fact that I don't know exactly when it occured so I can't be sure how far gone she is if indeed she is!
One minute she looks like she has swallowed a balloon, the next she looks fairly normal. I am really going mad! So is there anyone out there that has dealt with pregnant queens that may have some way of knowing?
I am going to sort the vet thing out on monday to get her checked out anyway but any ideas would be nice.

Thanks.

If she will let you, take a good look at her nipples. This is only applicable with a cats first pregnancy - the nipples will turn a bubblegum pink color on the 21st day (and then remain that color)...so if they are that color she is at least 21 days along out of what is on average a 65-67 day gestation.

I'm glad to hear she's going in to see her vet on Monday!