It sounds like you’re overtraining. Your body is conserving energy and shifting all the muscle nutrients to your legs because they need it the most right now. I **** to say it but you’re going to have to balance it out somehow. Lift some weights, you don’t need much and cut back on the bicycling. Make sure you’re getting plenty of protein and complex carbs right after your workouts. That’s prime muscle re-building time. And, if you’ve noticed loss of muscle mass you’re not losing just fat. Some of that is muscle as well. You might gain a little back on the scale but it won’t be fat.

Because cardio activities burn muscle.

Of course your leg muscles are developing though because biking involves mostly those muscles.

Eat protein to maintain your muscle. Try and use your arms more if you can, you don’t always have to lift weight — Lean on a wall, swing from a pole, etc.

You need to exercise you upper body as well, bicycling only builds the lower body.

biking works the lower body, mostly your legs, not the upper body