Could be a blood clot. I would get her to the ER for evaluation asap. Check her skin on that foot for warmth to touch and the color of that area- if it’s looking much different from her other leg, call 911.
Sorry to hear this. Does your mom's insurance have a 24-hour nurse line you could call for advice? Otherwise I would call her primary care doctor's answering service and talk to the doctor on call. I'm not a doctor or nurse, but if this just happened, I think you need to check in with a medical professional now and get their take on it ASAP.
Check her skin on that foot for warmth to touch and the color of that area- if it’s looking much different from her other leg, call 911.
Hope you can get some help for this situation.