Yep, gotta get back on the horse...er, bike.

Are you generally staying well-hydrated? I've found that not drinking enough water can cause that rubbery feeling in my legs. Of course you're not going for long, long rides yet, but it may be that you're just not drinking enough water in general. Anyway, it's something to consider. As Pooks said, go easy and most of all, have a good time!