osteoarthritis knee pain

If someone has knee pain does it always means that it is osteoarthritis, arthritis?

what could be other reason of the pain as it is only when the knees gets folded like sitting down on going down to pick up something and when i get up after.

No.
From the Mayo Clinic:
“If you’re suffering from knee pain, you’re not alone. Almost one in three Americans older than age 45 reports some type of knee pain, and it’s a common reason that people visit their doctors or the emergency room.

Knee pain may be the result of an injury, such as a ruptured ligament or torn cartilage. Or, certain medical conditions, including arthritis, gout and infection, may be at the root of your knee pain.

Many relatively minor instances of knee pain respond well to self-care measures. More-serious injuries, such as a ruptured ligament or tendon, may require surgical repair. ”

Their symptom checker is here: http://www.mayoclinic.com/health/symptom-checker/DS00671/SYMPTOM=43E2A918%2D2A5D%2D9994%2DE7C9F2A8BD7DC6A8&TAB=Knee%2520pain

: ) Good Luck.

New osteoarthritis knee Pain Research Study