I have great dental hygiene and i floss, but i noticed a thin black line near the gum on the back of my incisor front tooth. my gum looks lifted. what should i do?

Needs to be seen: It could be a number of things and would have to actually be seen by a dentist. It's unlikely it's anything to be concerned about. We can't make a diagnosis over the internet, especially without seeing what you are referring to. Perhaps a stained groove or calculus? When was you last dental exam and x-rays?

Evaluation Needed: Without an examination it's difficult to determine a diagnosis. It could be staining or other possibilities.

Could be stain: Could also be a thin piece of calculus that has darkened. When as your last dental check up and cleaning? If more than six months, now is the time to pick up the phone and make a call.

See you dentist!: It is impossible to really tell what the diagnosis of your described problem is without doing a thorough and comprehensive oral exam. Your "gum" being lifted and black line doesn't sound within normal limits. I'd suggest you see your dentist and have this professionally examined. It could very well be an easy fix or you might catch a serious problem early! good luck!