I've Been Running A Low Grade Fever For The Last

I've been running a low grade fever for the last two weeks. Doctor's have run blood tests and they all come back negative. No congestion. Usually in afternoon is when it rises up to about 99.5. Have taken antibiotics and steroid to no use. If I do a little physical exertion it immediately goes up. I've tried resting, but still run the temps. What could it be?