Rash on knees

Hi,
  About a month ago I had pneumonia and with that I had a rash.  That would appear in the form of hives at night.  I went to the doctor and was given medication for the pneumonia and the hives stopped as well.  Now at night while I am sleeping a red rash that also appeared at times with my hives is starting to appear on my knees.  When I get up and have been up for about 30-40 minutes the rash disappears, then it will come back again the next night.  I have not changed the foods I am eating, detergents, and change the sheets on my bed frequently, what could this be?  If it continues I am going to go see my doctor.  I also have psorasis, however this is not scaly, dry and bumpy like my psorasis.  Any sugestions as to what this might be would be very helpful.

[melw1982]

When I got up this morning the rash was on my knees but also on my feet.  It is red, flush with the skin and a bit itchy.  However it also disappeared after being up for a while.  I was feeling very fatigued but today I feel rested.  Any thoughts or insights as to what this might be?

[melw1982]

Rash went away for about a month but is back again now at night.  In the form of hives and then they go away when I get up.  Anyone have any insight as to what this might be?