Oops. I hate it when that happens. But it happens to all of us.
Just to make sure I follow you correctly, you guys were clothed and just making out or something, right? You said cuddly, so I’m going to assume you weren’t straddling more than just his lap when she walked in. If I’m right, it sounds like no big deal to me. So she caught you and her son snuggling and kissing — big deal. She already assumes you do those things, if not more. You’re both adults now, aren’t you? So her only real beef — if she has one — is that you’re doing that in her house. Otherwise, what you do is your business.
How did she react when she caught you? What did she say to her son later (if he told you)? That would give you some idea of how she feels. If she thinks you’re a hussy, then she thinks you’re a hussy. She likely formed an opinion of you the first time you two met, and catching you sitting in your son’s lap isn’t going to change that. You can’t control what she thinks of you, and if you try by kissing up to her, it will just make things worse. She’ll wonder why you’re trying so hard.
You can’t really save face at this point, so just stay away from his house until the embarrassment wears off. You guys need a better make-out spot, anyway. Even though you were just sitting in his lap, most parents don’t like the idea of their kids fooling around in their house when they’re not there. She knows you two are physical, but she doesn’t want to see it. If you continue to do it right under her nose, you will get caught again, and then it becomes a matter of disrespect.
Don’t you have a dorm room? That’s the place to conduct your dirty business. Or do what the rest of us had to do — make out in the car or the bushes or on a golf course at night. You’ll get caught again, but at least it won’t be the guy’s mom.