When i floss, next to one of my teeth, there is a sensitivity around the gums. what does this mean?

Inflammation: It could mean that you are flossing too hard or it could mean that there is some inflammation in your gums from calculus and bacteria. If you have not had your teeth cleaned recently, you need to do this now. See you dentist.

Root exposed/cavity?: Sensitivity upon flossing can be a sign of root exposure, the start of a cavity, or even flossing too enthusiastically. Please contact your dentist. The cause will probably be evident upon examination.

May be ok: Occasionally, floss may cut into your gum and cause sensitive or pain. However, if occur to the same area or the same tooth, it may indicate a chronic food impaction causing localized irritation of the gum chronically. Close the contact in between teeth if there is an open interproximal contact. Clean areas thoroughly.