Im sorry to hear about your symptoms.
There are a couple of things which can cause this sort of pain behind the knee joint. Now you havent mentioned any history of trauma at the site so im assuming that is not the case here. So the first thing is that you could have bursitis which is inflammation of a fluid filled sca inside the knee joint. It could cause pain of this sort and it can also cause a swelling as well as redness in the area. This can be diagnosed on a clinical examination as well as a MRI of the joint. This is usually treated with the help of OTC NSAIDs like motrin or aleve however in some steroids can be used.
Another possibility is that of a bakers cyst which again can cause this sort of pain. Now usually a bakers cyst does cause swelling behind the knee joint and the pain worsens when you stay active and after activity. This can again be diagnosed with the help of a simple clinical examination and again treatment is with the painkillers i have described above. Howevere in most cases the fluid has to be drained.
Now there are several other conditions which can cause this pain, ranging from osteoarthritis to a ligamentous injury inside the knee joint or even a meniscal injury. As you said, this would need to be evaluated at your doctors office and with xrays as well. But the best test in this scenario will be a MRI as that can yield the most information. That being said, most physicians can easily make a diagnosis in most causes of knee pain clinically as well.
now as to what you can do right now, Rest is essential. Keep the knee elevated if you can and at the same time you can try and Ice the area which is causing the most pain. OTC drugs like motrin and aleve can help in reducing the pain as well. However since you havent mentioned the drugs which you are on (if any), the safest drug which you can take for the pain is tylenol. If you are not on any drugs currently or have any medical conditions, then motrin and aleve should be safe as well.
I would also suggest that you do contact your doctor and have him look into this before the pain gets any worse. Here is more information on the two most likely possibilities.
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