You can make great nesting areas but unless the cat is contained somehow, it might decide to have the kittens anywhere. From the sound of your story, I would bet she is having kittens. A veterinary visit could confirm this but the time of year and age of your cat make the answer rather likely to be yes. If there is anywhere to contain her such as tack room, shed, or enclosure meant for a different animal, I'd put a litter box and food and water in there and keep her contained until the time comes.
Please think about getting mama and babies spayed and neutered, there are already too many cats in the world! You will quickly have a large population of cats unless you intervene.