One issue with concussions is that we don't truly recuperate from them for many reasons. We can have "concussive events," such as the loss of a loved one, that negatively affect our brain. When we don't adequately rest or nourish our body, then these concussions or concussive events can pile on one another and cause issues down the line.
Getting POTS after a concussion is just one of many possible negative scenarios:
https://coloradointegrativeneurology.com/neurological-health/have-you-suffered-since-a-concussion-you-might-have-pots/
If the doctor said to stop, I'd stop. Why would you not listen to the doctor about this?
It's terrible that they don't wear helmets in lacrosse to prevent this, and that should be changed to protect people's lives. Because people don't die, they don't do enough. However, having a life-long illness like POTS or other is serious.