From what you have described, I understand that you have pain in the knee, esp. on the inner side with swelling, after an injury 3 weeks ago.
It seems that you have an injury of the soft tissues in the knee, maybe ligaments, cushion(menisci) or cartilage. Since it has already been 3 weeks, it would be a good idea to get an MRI knee done. Xrays show bones only, while MRI shows all soft tissue structures as well.
Next Steps
Consult an Orthopaedic surgeon
Get an MRI of the knee
The Orthopaedician may suggest physiotherapy, and maybe some blood tests.
Health Tips
Do not perform floor activities, sit cross legged, or sit on low chair/stool