Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain it seems that sharp piercing pain in the area in between two lower back teeth and occurs when the floss touches the gums..
Well, it seems that the pain is not related to any of the tooth, rather it occurs due to inflamed gum in the area..
At times due to vigorous flossing the tip of the gum continuously gets injured and it leads to its inflammation and pain when the floss touches the area again..
Second reason can be food
impaction in that area leading to infection, inflammation and pain..
I would therefore suggest you to avoid flossing the area vigorously for a few days..
You can apply an antiseptic oral gel like Chlorohexidine in that area and
massage gently..
Do cool compresses over the gums..
You can do warm saline gargles..
In case if it does not resolve in a week consult an Oral Physician and get evaluated..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.