Thanks for the query to H.C.M. Forum.
You have not mentioned that pain is in one knee joint or both. Rash
appears after knee pain
or after any medication for knee joint.
Since rashes spread also over arms and more over body parts .
Here we require few things before any diagnosis as,
Reports of C B C, E S R( INFECTION), R A factors( rheumatic factor), uric acid ( gout
), any old trauma.
May be due to osteoarthritis
but for this you have to consult an Orthopaedician and get his opinion.
For rashes you have to consult Dermatologist
Hope I have answered your question.
If further any question I will help.