You didnt lose muscle, just some of the water/glycogen i would guess, and the swelling from all that overtraining damage you were probably doing!
I expect you wouldnt have gained much actual muscle in month of arm specialisation.
Anyway, dont worry.
You keep gained muscle a long time but it will dwindle if you dont train or if you eat too little - you dont have to train as hard to keep it as to get it by all accounts but train you should. If you dont enjoy the journey you dont get to stay at the destination!