my cat is acting strange... instead of running around and being active she has been sitting in the same spot for most of the evening.. also she has not eaten any of her food... should this be a concern?

Definatly a concern

Maybe a good concern
Maybe bad

If shes an un-neutered outdoor cat , im thinking . . Pregnant ?

How long has that behavior gone on for? Usually in animals the first sign that something is wrong is they stop eating. In SOME animals, is the ONLY way you can tell something is wrong. They're very good at keeping symptoms hidden, it's a defense mechanism. If it's been going on for a day or more then you need to take her to a vet ASAP, if it's just tonight that she's been acting weird, I'd keep an eye on her til tomorrow. Put a small amount of food in her bowl, if that's gone in the a.m., then she ate and she's probably fine. If it's still there in the morning, then make a vet appointment.

Cat owner most of my life.

Yes