Thank you for the additional information.
There are several comments that are pertinent to this situation.
First, if there is tenderness in the area, that would indicate that it is most likely musculoskeletal. Yes, it can occur from an injury as your doctor stated, but it also could be due to muscle pain or musculoskeletal inflammation. In this area, the inflammation would most likely be a tendinitis or bursitis.
There are certain viral
infection that can cause musculoskeletal pain, some cause muscle pain while some can cause inflammation. For example, parvovirus infection can cause musculoskeletal inflammation, most often as arthritis.
It is important to note that there is no sexually transmitted disease that will cause knee pain that develops in 4 days. I understand that you noticed the symptoms shortly after intercourse with a new partner, but since there is no sexually transmitted disease that can cause these symptoms, there is no reason for concern about such a disease.
From your description of symptoms, it sounds most like a musculoskeletal issue, and it would be reasonable to initially manage it as such. This can be done with the use of over the counter anti-inflammatory medicines, such as ibuprofen or naproxen, and local care, including ice and moist heat.
If I can provide any clarification, please let me know.