Does your GP mean inflammation bloods like ESR and CRP or slightly more specific like RF factor? Some people have RF factor without having rhumatoid arthritis (or at least symptoms) and it can go up and down, same with the inflammatory markers.
I don't think I had an RF factor originally on some tests but do now and my inflammation markers go up and down though are usually raised.
There are many different types of arthritis and similar conditions, loads of them! Some have different blood markers and levels can go up and down.
What are your symptoms and how long have you had them? If there is a suspicion of something like rhumatoid arthritis and you have symptoms then have they referred you to a rhumatologist?
Doesn't mean to say it is though, my GP was convinced I had glandular fever a while back and based on test results but more specific tests came back negative.