Oh dear Ii can understand your fear. But see there are plenty reasons for fever. First diagnosis of that fever and treatment is necessary. If it was protected sex, no need to worry at all if condom was intact. If you are in contact with prostitute , you can reassure by check up of her that she doesn t have any infection. Your current tests for HIV negative, so just relax yourself. If still thought persist in your mind and affect your daily routine and work, you can take help of psychiatrist to prevent OCD or post traumatic disorder like situation. You can be helped out through counseling and/or medicines.