I agree with the rest of the posts. . .take a week off.
I actually have had a similar experience, and just went back to the gym tonight after 8 days off. Let me tell you my experience. . .
When I first started working out about 1.5 years ago, I used Arnold's Encyclopedia of Bodybuilding as a guide and was working out 6 times a week, each body part 3 times a week. My split was something like bis, tris, shoulders, back one day and legs and chest the second day.
I did see some initial results, but not as much as I would have liked, so after about 8 months of training like this, I went to a 3 day split training every body part twice a week. My split was back and chest one day, bis, tris, and shoulders the second day and , and legs and forearms the third day. Still six times a week.
Looking back on it, I was stupid. I was overtraining my body. I very rarely got great pumps or adrenaline rushes from working out. My body always felt how you are feeling. . .I would get tired and not be able to finish a set, but the actual muscle I was focusing on wasn't fully fatigued and wasn't feeling a pump.
Well a few weeks ago I decided to take some time off and switch my routine so that I only workout each muscle once a week. I just went to the gym tonight and did biceps and triceps. It had been eight days since I had been to the gym. My arms exploded. It was insane. I think I lifted less than I could cause I was in shock. So happy that I finally got a pump back. It has been an hour since I got back from the gym and my arms are still feeling hard.
Moral of the story: I am by no means an expert, but from a little bit of failure of my own, I can say this: Listen to your body. If you are no longer getting pumps and that adrenaline rush, you are probably overtraining. Take some time off. Don't think twice about the fact that your arms or body is getting smaller. It is not. People that live in the gym and overtrain get addicted to the idea of their muscles being big. So when they take a few days off and their muscles dont feel big, they think they are shrinking. Not true!! If you love being at the gym, do what I have decided to do. Split your workout up even more. Do a 4-day split. Do a 5-day split. With just one or two muscles per workout. Try it! I think you will be happy with the results.
Sorry to be so lengthy. Was just excited about getting my pump back tonight.
My two cents,
Cajun Bob