Why does exercising increase my anxiety? isn't it supposed to do the opposite?

It usually helps: Exercise is often recommended for anxiety relief. But everyone is different, and in some cases it might worsen the problem. Here are two examples: if exercise is associated with past trauma/stress (say from an abusive school coach). Or if a fast heartbeat scares the person that they may have a heart attack. There are many other possibilities as well, which is why "one size doesn't fit all.".