did you do pushups in the mean time, while you were working out? or were you only doing pushups and then moved to lifting exclusively? if its the latter, i dont see this as a big problem..your muscles that push u up and down are strengthened specifically, and if you start working out and not doing pushups, you may not be maximizing those muscles the way you did before...this is an addition to things that have been mentioend before...
i have had the same thing happen to me...i used to always wake up and do 50 pushups, but when i started working out, i stopped doing them..when i stopped working out and went back to pushups, i was unable to do what i did before starting to work out, even though i was prob. stronger..
hope that helps