Well from what you are describing there are a few conditions which can cause symptoms like these. The first one which comes to mind is that you could have developed bursitis which is a condition in which there is inflammation of small fluid filled sacs inside the knee joint. These can sometimes be pushed outwards from the joint to under the skin. These bursae can be inflammed because of trauma or even an infection. In most cases this can be diagnosed on a clinical examination and a xray and if needed a MRI of the knee joint can be done as well. The treatment is with OTC anti inflammatory medications like motrin or aleve as well as topical agents like voltaren gel. If needed a steroid injection can be used which is extremely helpful.
Another cause of this can be Patellofemoral syndrome. Individuals with this condition have pain under or around the kneecap. The pain is worse with flexion or after sitting for a long period of time. The American Academy of Family Physicians recommends that individuals who are experiencing this overuse injury take a break from their activities, ice the knee several times per day and evaluate the type of footwear they use. Exercises can also help to strengthen the muscles around the kneecap and relieve the pain.
ANother cause can be a baker's cyst. This cyst is an accumulation of fluid that forms behind the knee. It can be caused by herniation of knee joint capsule or the tearing of the meniscal cartilage of the knee. Sometimes the cyst may be displaced more towards the medial side and again this causes pain more on flexion of the knee joint. The treatment is with steroids and if needed surgery.
There are several other causes for pain on flexion of the knee joint with a lump such as iliotibial band syndrome etc however i would suspect the above three as they are most common ones, especially bakers cyst and bursitis. I would recommend that you start using OTC motrin and aleve for the pain right now and apply topical voltaren gel as well. But do get a xray done of teh knee joint and a MRI as well by visiting your doctor. This should be evaluated sooner rather than later.
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