Could my cat be pregnant?

I have a 10 month old indoor cat who is not spayed. A few days ago my brother accidentally let her out and I didn't realize until 4 hours later. When I found her she was with a neighborhood boy cat sitting on the porch. She has since stopped meowing almost completely and just lays around and purrs (she is usually very vocal and aggressive). Could she be pregnant? Would the boy cat have stuck around if they had already had sex? I really want kittens so it may be wishful thinking, I'm just curious.