Thanks for your query at DoctorSpring.com. I have read your query with diligence.
It looks that you are having knee pain which you feel when you stand after sitting for long or short time. This problem is generally encountered with either meniscus injuries or with abnormalities at patella or knee cap bone. At present you need a MRI investigation to detect any early abnormality at your knee. This can be a meniscus injury at your knee which can be due to presence of overweight, which you had previously. If your meniscus problem is minor then this can be corrected by strengthening exercises of quadriceps, if it is major then this should be repaired by surgery.
Taking X ray is not necessary as this will only show bony abnormalities.
Hope this all will help you. Please feel free to discuss further.