If she has been outside she is almost certainly pregnant; male cats do not take no for an answer. Literally (I've even read the word 'rape' when it applies to male cats impregnanting a female cat.)
There are other signs regarding how pregnant a cat is, like can you see babies move, any milk discharge. Here is an article to get you educated on the subject and here.
I would start saving your money: it is very expensive to feed a pregnant cat and kittens, not to mention vet care. While female cats kind of know how to give birth, it is very very expensive if things go wrong and the cat has to be taken to the e-vet or your regular vet (much more expensive than a simple spay.)