Your body is adapting itself to the lower demands you are placing on it. There are two basic types of adaptation that your body can go through when lifting weights:
- Myofibrillar hypertrophy--this increases the number of protein pairs per muscle cell. In very basic terms, the more protein pairs the more work that muscle cell can do. More myofibrillar hypertrophy translates to increased strength--but not necessarily bulk.
- Sarcoplasmic hypertrophy--this increased the amount of energy support within each muscle cell. In very basic terms, it allows you to lift for longer periods of time. The energy support systems take up more room than protein pairs, and is responsible for the bulk that bodybuilders seek.
When your body is detraining itself due to inactivity for longer periods of time, the first systems to unadapt themselves are the support systems. It holds on to the strength part just in case it is still needed, but the body assumes that the need for that strength will be fewer and farther between. This is why it is always a good idea to either repeat your last training cycle after an extended rest or take a small deload.
In your case, the sarcoplasmic hypertrophy that you attained while working out is starting to become unadapted (less sarcoplasmic fluid in the muscle cells). The myofibrillar hypertrophy takes longer to unadapt (or adapt to lower levels).
Bottom line: the longer you go without training, the more you will lose endurance/size (first) and strength (second). However, because your body knows how it needs to adapt when you start training it again, you will get back to your trained state quicker.