I am very happy that I have to write so seldom about it, but sometimes it even happens to me. I think I underestimated the intensity of the second series. Muscles are used now that weren't used so far and I have obviously injured shoulder or pectoral muscles. I do not feel anything when I move around. I only feel the pain when I put the right leg behind my head. Non-yogis would probably say: So why do you put your leg behind the head? And for a while it might be what I have to do. The pain is unbearable and feels unhealthy when I try dwi pada sirsasana (both legs behind the head) for instance.
I think whatever you do injuries can happen: skiing, soccer, running.
To be attentive helps in yoga to avoid injuries. But sometimes it is simply not possible to know the own limits. Now I have to step back. The pain that really hurts is my guide. It tells me exactly where to stop. Great.