Why don't you try a few days without chewing gum and use the Wrigleys Extra Strips instead as a breath freshener. Both my LLC and CDCs said DO NOT have chewing gum as it stimulates digestive juices and can throw your body out of ketosis. If after 3-4 days you're still not in ketosis then I guess you would need to look into it further.
Out of interest, have you always tested neg or have you been pos before? And if you have been pos, were you chewing gum then?