Thanks for the query to H.C.,. Forum.
Since you mention that lungs and heart functions are normal .
The swelling over feet , below knee are the main reason of joint pain if only related to knee & ankle joints.
1 If pain in other joints are also present then get in blood for rheumatoid factors.
2 Since you are a female of 41 years hence estrogen
level depletes so get in blood for calcium level to rule out osteoporosis
which is most common .
3 The most important cause of swelling below knee may be due to involvement of kidneys , so kidney function test are important , please get in .
For confirmation get in ultrasound
of both kidneys.
Consult a physician and get his opinion.