Knee pain is a common problem among the young and old alike. From athletes to middle-aged adults to seniors, knee pain can develop suddenly. There are many potential causes owing to the fact that there can be ligament involvement, cartilage tears, muscle strains, cysts, arthritis, and more.
Most of the time, knee pain is felt in the front of the knee or along either side. Posteromedial pain (inside back corner) like what you are describing is less common and more puzzling.
One possibility for the symptoms you describe is a condition called semimembranosus tendinopathy. The semimembranosus muscle is part of what you might know otherwise as the hamstring muscle. It is made up of three separate but conjoined parts. This portion starts at the base of your “sit bone” (called the ischial tuberosity).
It travels down from the pelvis to the knee and inserts right along the posteromedial corner. The job of the semimembranosus is to flex or bend the knee. You may be feeling this tendon. It is palpable when you touch under the back of the knee on the side closest to the other knee while in the sitting position.
When musculoskeletal problems of any kind develop, it’s always a good idea to have an expert (your physician, a physical therapist, or other health professional) take a look and advise you. This can save you time, pain, and money in the long run.
It’s often the case that the longer we put something off, the more complicated it becomes. Other areas of the knee get involved and instead of one problem, we end up with a combination of difficulties.