Perhaps you didn't lose muscle- maybe it was fat.
Unless you had a bodyfat caliper analysis done before and after your haitus, I wouldn't necessarily assume you've automatically lost fat.
Besides that, if you were overtraining, and you finally get the rest you need, you'll find yourself stronger coming back when your muscles finally do get enough rest. In a good periodization program, there is always a rest factored in after your heaviest training sessions come to an end. When you come back and get back into it, if you've structured your periodization program correctly, you'll see the positive results in your training.