Welcome at HCM.
It looks from your details that you may be suffering from arthritis
of knee with associated ligament involvement.This can be due to lack of exercises or increase in weight or old injury
You should do following to have relief in this-
-Consult to a orthopedic doctor for this.
-Get a x ray done.
-Start doing quadriceps exercises-
-Take a good anti inflammatory analgesic
like advil or aleeve.
-Take joint supplements like glucosamine
or chondroitin sulfate.
Hope this will help you.