Starting to Atrophy?

I joined a new gym.

My exercise of choice is running. I use that term loosely because the speed at which I move at can barely be categorized as running. But it is what my body currently recognizes as the act of running, so that is what I call it. I can't run outside in the winter though, so I broke down and joined a gym that was having a short-term membership special to get me through the winter. And I am really enjoying it (as much as one can enjoy dressing in unforgiving spandex and sweating in public).

After a week of shaking my stuff on the treadmill, I braved the Nautilus equipment for some work on my upper body. Specifically the waddle under my arms (my girlfriend calls this her "Hi Toby" after the charming effect she noticed when frantically waving from the audience at a Toby Keith concert). My pseudo-boyfriend Robert was there working out with me, so I think I may have been showing off a bit, but it felt good...at the time. Two nights later I awoke at 3am with the feeling that the circulation had been cut off in both my arms, or that I had somehow suffered nerve damage by sleeping unknowingly in a challenging yoga position. I finally figured out that this must have been the result of my ridiculous display of strength and stamina, and have been suffering silently ever since.

Is it possible for your muscles to atrophy from lack of use? Doesn't carrying a 50lb 4-year old around regularly count for anything?