MaxMax is exactly correct. Just to clarify, while it isn't a bug, you really want to avoid having negative resists. You will be very vulnerable to elemental attacks. Try to improve the resists by changing your gear and charms. You should try to get your resists at least into the 30's at the start of nightmare.
I made a charm that gave me 30 life and resist 16 to all. Should I also make 2 rings or would that limit me with the other stats my rings currently have? I have a killer shield with +80 resist fire but I have an even better shield with less resists. I have been getting killer drops and most of my equipment is unique.