I just went through this with my dog.
As some of you know I was blessed with a litter of 7 puppies this last month. One main way to know if there's a possibility of your dog being pregnant is if you know for sure she hooked up with a male. Secondly they go into heat for 2-4 weeks. If during any of that time she stops bleeding ahead of schedule you can pretty much guarantee puppies within 63 days. The first time my dog, Daisy went into heat she got pregnant on accident. We weren't trying for puppies at all and all it took was one hook up.
One of my cats was like that before I had her fixed, though. She wasn't overly friendly with anyone besides me and the kids, but watch out when she went into heat! We used to laugh because the minute Mini's boyfriend came over she was all over him and if he took his shoes off she would roll around all over them like they were the best things ever.
Animals are weird. ;)