I have most of the symptoms of sarcoidosis such as fatigue, erythema nodosum, shortness of breath, but the test I took for sarcoidosis came back negative. Why the erythema and other symptoms?

Many causes! Erythema nodosum may occur as an isolated condition or in association with other conditions.Conditions that are associated with erythema nodosum include medications (sulfa-related drugs, birth control pills, and estrogens), strep throat cat scratch disease, fungal diseases, infectious mononucleosis, sarcoidosis, behcet's disease, inflammatory bowel diseases (crohn's disease and ulcerative colitis).
Other diagnosis. The results of the test could be a false negative. More commonly, another disease is to blame. Check out the following website: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0001884/. You need to see a good internist that can complete the workup and determine the diagnosis.