It could be that you are running into muscle exhaustion. You don't say how many reps and what weights you are doing. I usually do the following:
a week of higher reps (12-15) at light weights, about 56-60%, example 25 lb
a week of medium reps (8-12) at medium weights, about 70-80%, example 35-45 lb
a week of low reps (1-8) at heavy weights, about 85-90%, example 55-60 lb
Then start the rotation again.
Every so often, I take a week off, and just do very light weights or something different.
You may not be overtraining your muscles, but it is possible to reach your bodily limit by not taking rests, or by not varying your program. By all means, go light for a few days.
I am just coming off a 3 week layoff because my shoulder seized up. Not wanting to fool around with a shoulder joint, I don't at all feel guilty about not working out. When I get back, I'll be right into the swing of things.
Hope this helps...
Do or don't do; there is no try (Yoda)