Seronegative means that there are no antibodies detectable in your blood stream that react in the standard ways that rheumatologists
use to classify the different types of arthritis
syndromes. Polyarthritis means that you have multiple joints that are inflamed or involved.
I would recommend discussing this issue with your rheumatologist. Although I don't know all of the information about your case, I therefore gather that you have a fairly significant degree of joint stiffness and pain that affects multiple joints. This probably has the features of a typical autoimmune joint condition, but your blood tests have come back negative. It turns out that a good proportion of autoimmune joint conditions fall into this category.
They can be a bit more difficult to treat, because the lack of antibody reactions in the bloodstream according to a standard pattern makes it harder to know which medications are likely to be effective. This often means more visits to see your rheumatologist while different medications are trialed to see which is most effective.
I would start by setting up a follow-up visit with your rheumatologist to discuss the issue. They should be able to tell you what they are thinking in terms or your available treatment options and plans for follow up.