Hello,
I have gone through your query and understand your concern.
At this age,
Osteoarthritis is very common.
First you have rule out any arthritis.
As you said there is no history of any
injury or accidental fall, there are some other causes which can also cause severe
knee pain.
You have rule out Backer's
cyst. it is one of the very common cause of the pain in posterior part of knee in females.
It is nothing but a Inflammation of the popliteal bursa (fluid filled sac) at the back of the knee.
your doctor will diagnose by simple examination and find a cyst like collection in back of knee.
please follow this to reduce the pain
1. ice the area with ice packs.
2. Take pain killers like Ibuprofen
3. Give rest to the knee.
If these does not decrease the pain, you may need aspiration, injection and
electrotherapy.
Hope this helps.
Please write back for further queries.
Wishing you good health.