Right Knee Started To Hurt A Week Ago , Hurts
Possible damage of the knee ligaments.
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I read carefully your question and I understand your concern.
What you are describing seems like damage of the ligaments that hold the knee together. It is typical for those type of injuries to have the pain exactly when people go up or down the stairs.
How much is the damage can be known for sure by doing an MRI and a visit to an Orthopedist who will evaluate if you have any instability in your knee...
I see that you do not recall any injury, but if you are having that type of pain, then for sure there was an injury sometime while you were on the go, and that you didn't pay attention in those moments. It doesn't hurt when it happens, but it hurts after when you start bending and climbing the stairs.
My advice is to protect the knee for some time and avoid those movements that cause the pain. Stay rested, keep the knee elevated. Apply ice pads, if that feels better. Take a painkiller to help reduce inflammation- Ibuprofen 400mg 3 times per day for 1 week after food (unless you have contraindications for its use).
If only a strain of the ligament has happened, then it will recover in 4-6 weeks. But if a rupture has happened, you will have instability, and will get worse- these are indications to go and see a local doctor to examine you and advice further examinations.
Hope this is helpful.
Let me know if you need more clarifications.