Hi, I Am 34 Years Old, Generally Healthy But Have

Hi, I am 34 years old, generally healthy but have been experiencing achy muscles around my knees. My legs feel often like I have run a long distance after I move after standing for a while. I have been taking 800 mg of ibuprofen at every meal. The pain radiates down my legs when I sit/move after standing for long periods. The pain was a sudden onset around Thanksgiving and is strongest at the knee joints particularly when at angles below 90 degrees. Kneeling down and getting up is painful. Related/unrelated I have been what I would characterize as excessively thirsty lately. What is this?