Even absolutely Champion athletes have like 4-5% body fat IIRC. And at that point the only thing that has fat on it is basically your internal organs. You shouldn't be losing muscle mass unless you are working so hard that your muscles are devouring themselves for energy. That is basically only going to happen if you're working out and pushing yourself way too hard. Somehow I really doubt you're pushing that hard at work on a daily basis. Maybe you're not getting enough protein. Increase your calorie intake, and focus on getting one gram of protein per pound of body weight per day. Remember your body can only absorb about 30 grams of protein in a sitting. Protein powder shakes are good way to achieve that. When working out I drink one about every two hours unless I'm eating a meal until i hit my goal protein. Start working out regularly, once every 2 days basically. Progressive overload is the key but don't damage your body. Form is more important than weight until you get the hang of it, but ultimately your gains are correlated with the amount of "work" that you do. I'd start off on the low side for the weights make sure your form is good, and do four or five sets of 12 reps on whatever workout you choose resting 30 seconds in between sets. Even with a lower weight you're going to be sore about 3 days afterwards. It's called delayed onset soreness and it is totally normal it will go away and shouldn't come back unless you're pushing really really hard. Basically you want wake your body up with a light workout, wait a few days for the delayed onset muscle soreness to kick in and then heal. Then you can progressively add weight until you find the edge of your strength. Workout at that edge for a couple 3 weeks and then add a little bit of weight. Remember, Progressive overload is the key. Oh, and You won't gain unless your calorie positive. Return successful!... OR NOT AT ALL!