I wouldn't worry too much about it. That most likely happened because you had disidentified with your thoughts. Being in touch with our core being (who/what we really are) brings about different sensations if one is not used to being in touch with it. I sometimes experience the numbness you speak about.
As for your mind drifting without you "knowing", you'll know. Awareness is what meditation is all about, so you'll know if your mind is drifting. If it does drift, don't worry about it, just return to your mediation technique.
And don't get used to any feelings that arise from mediation either. Every single session is going to be different in it's own way, and that is great because there are no expectations to live up to.
