You're not givng up a helluva lot of information but there are some basics to follow...
To answer your question - Yes you can lose muscle if you are over-training (or doing too much cardio), not resting, or not consuming enough calories.
Secondly... Are you sure that you're losing muscle and not fat?
Third... Your body will get into a rut and will not respond to weight training as well if you follow the same routine(s) day in and day out.
Youhave to mix it up and incorporate diffferent techniques to "shock" the system and respond by muscle growth.