in personal experience, if i train 3-4 days a week, the one week i'm pretty strong and then the next week not as much - almost like a 2 week cycle, so this might be what you are experiencingFunny thing is I seemed to be getting weaker each time and surely this shouldn't happen at such an early stage?? even if I do eat plenty...well at least I think I do.
also ensure that give the body part that you trained enough rest 48-72hrs