Coming from someone who weightlifted five days a week prior to my injury I would suggest that you NOT lift any sort of weights. Lifting weights has caused me to have relapses. It took me two months of back and forth feeling better, exercising, then relapsing before I realized that it is not worth it. Yes, I have lost some muscle mass too, but who's to say you can't gain it back when you're all better? It is very difficult for me not to exercise, but I realize healing my brain is more important.

It is probably OK to walk and do some light yoga/stretching, as these activities are not very demanding but will still give you the satisfaction of doing something and it is good for your body. This is what I have been doing for about 2 weeks now and I will continue to do it until I see a specialist.

Unfortunately a concussion is a test of patience. I no longer have headaches yet I still haven't went back to weightlifting because I still have other symptoms such as anxiety, foggy thinking, fatigue, and dizziness. Ideally you should be symptom free for 2+ weeks before even considering to lift light weights and ramp up your physical activity. It sucks, I know. But in the end when you recover faster you will be thankful you made the sacrifice. If I could go back to the day after my injury I would have quit all demanding physical activity from the get go.

I can only offer advice based on my experiences. Good luck on your recovery.