I've had a low-grade fever for 3 months with esr>100. every test under the sun (infections, connective tissue disorders, cancer) has turned up negative. is it ok to just ignore & wait and see now?

Not ignored: Are you on an estrogen preparation ? Estrogen can increase the ESR although not to the degree noted. Since this points to inflammation, you shouldn't ignore it . Make sure that your doctor is keeping an eye on your condition. If your headaches are really severe, check for temporal arteritis as well.

No, you shouldn't. : This elevated ESR suggests an ongoing inflammatory response, and in the absence of a known infection or acute tissue damage and repair, i'd be concerned about an autoimmune process. This is all the more likely in an adult woman. You should work with your primary physician to find a source for this problem. Your care may ultimately require referral to a rheumatologist. Good luck. Keep us posted.

Chronic illness : Low grade fever and ESR greater than 100 is of concern and could be due to several reason including occult sepsis, inflammatory disorders, malignancies, sarcoidosis, tuberculosis to name a few . You need to be thoroughly investigated .You seem to have detailed work up with negative results .Do consult with a rheumatologist and an infectious disease specialist for further detailed evaluation.