You might like to post on CFS Ask and Advice board though it's not very busy.
There are Viagra Forums on Google such as
http://www.medhelp.org/HealthTopics/Viagra.html which may help.
The sort of sensible advice is to consult your Doctor but I suspect that many are not comfortable with that.
You should at least try quitting the Viagra and see what happens.
I wonder if the natural libido urges both mental and physical are affected by using Viagra.
Our Moderator has sharp knives. I wonder if this post will last.