After a month's hard work, I took a video of Sanda today and found that the punching speed was twice as slow as before, and the movements were much stiff. What should I do?
I guess you were too busy working while the iron was hot to relax your fighting training. In this case, your body may not be used to the muscles that become heavier because of being more developed, or your muscles may not be able to give proper feedback to nerve signals. Hurry up to increase training and let the body gradually adapt to the current state.