Had arthroscopic knee surgery 2 mos ago but fell 2 days ago, and painful swelling, could I have reinjured it?

Yes, but probably no. I tell all my patients how to protect their knees after surgery often with bracing and/or crutches. My guess is that most fall at some point in the early postoperative period. So you are in good company. It really spends on two things: what exact surgery was performed, and how bad was the fall? You should check with your surgeon to see how concerned he or she is. A quick check alleviates anxiety.
Yes. You need to see your orthopedic surgeon for reevaluation.