A vicious circle has begun. Anxiety created because it happened before, breathing goes to pot, which creates more anxiety.
Screampillar is right.
I had no belief that I could run faster after my asthma diagnosis, until I went to a track session and tried running fast in a safe environment that meant if the worst happened that there were people around and no more than a few hundred metres from help. The lungs hurt from fast deep breathing but breathing was possible, I had to regain fitness.
The paper bag is because you were/are hyperventilating, often caused by anxiety.
Try going for a very slow run. You may have tried going out at your usual speed but being post half marathon your body may have still been tired or possibly picked up a virus. Add in that you have now formed a habit. a very slow run that is almost walking pace will not need a huge amount of air and get you back into a healthy habit.