Core Strengthening Exercise- Straight Leg Raised With Toes Turned Outward, repeat 10 times, twice a day. Hams Stretching- lie straight, take the leg up, pull the feet towards yourself, with a elastic tube or normal belt. repeat 10 times, twice a day. Quadriceps Exercises- Lie straight, make a towel role and put it under the knee, press the keen against the role, hold it for 20 secs. Repeat 20 times twice a day. This will help relieve some pain.

You got to have a proper clinical evaluation at the earliest..... Even though it sounds like a simple degenerative issue..... It could be much more than that.... Even though you give no history of injury and a degenerative ligament tear is uncommon at your age it needs to be ruled out..... Keep your vitamin D at level and visit an orthopaedic consultant near your area soon

At first start taking vitamin D supplements till it comes to normal level,besides that give enough rest to the knee joint by avoiding activities which causes/increases pain like squatting,jogging,climbing stairs etc,You might have unknowingly hurt your knee or there could be early degenerative changes in the knee joint ...so better would be to get X-ray done to rule that out...do icing 2-3 times a day...also you can use a knee support or a crepe bandage to protect it...Visit a physiotherapist and get it examined properly and start with the knee strengthening exercises.