Thanks for your query.
Considering your age and your present complaints, you might be suffering from arthritis
of patella. In early stages it presents as painful initiation of movement of knee and later on subsides during routine work. This condition is called as Chondromalacia patellae
. But it is too early to say that you are suffering from this condition, without a proper clinical examination by an orthopaedician. Visit your doctor, get yourself examined. Your doctor will advice you to go for a xray.
for your quadriceps muscle with straight leg raising and knee mobilisation will improve your muscle strength and will decrease the progression of arthritis. Life style modifications like avoiding stairs, squatting, sitting cross legged & cycling will also help in reducing your symptoms. Reducing weight is of prime importance if you are overweight
. If the pain is severe, you can take few doses of NSAID's or physiotherapic modalities like IFT to relieve your pain.
I do hope that you have found something helpful and I will be glad to answer any further query.