Most anti inflammatory drugs can take a few days of taking them to work, even the likes of ibruprofen when used for a few days build up their anti inflamatory effects.
You say that the pain is all over your body, is there any swelling on the small joints like your fingers and knuckles at all? As Ramrod has stated Rheumatoid Arthritis can have negative readings in blood results, yet folks can still have it (again not suggesting this is what you have) and I can only back up what Ramrod has said that if your GP has exhausted his expertise he should refer you to the rheumatology department.
Hope you're feeling better soon
Jo.
(Sounding like Talos from jason & the argonauts for 16 years
)