It is impossible to say for sure what is causing your knee to be suddenly stiff and painful without more information, which is why you should see your family doctor
to answer this question. The most likely cause of knee pain would be an injury, and it is possible that it was something that happened quickly and was so small that you hardly noticed it while it was happening. The fact that you mention that you are not physically active makes this more likely, as taking any misstep walking up the stairs or a curb could be enough to pull a muscle or strain a ligament. This sort of pain usually responds extremely well to non steroidal anti inflammatories (NSAIDs), but you say that you have been told not to take medications like ibuprofen. The next thing to be recommended would be non-painful stretching and cold ice packs to reduce the inflammation to the knee. If you have tried these already and nothing is helping, then you will need to see your family doctor. He or she will be able to do some simple tests in their office, possibly (but not usually) including an x-ray, and then offer recommendations. They would also be able to tell you if it is something other than an injury, as there are other causes of knee pain as well. An orthopedic surgeon
would also be able to help you, but a family doctor would likely be the place to start. Regular exercise and stretching will help with bone and joint pain in the future.