Can flossing cause a cold sore outbreak? like if you do it too rough just to get the plaque build up out?

Flossing can be done too vigorously, causing injury to the gums which is painful, but will not cause a "cold sore" (herpes I/II) outbreak. Herpes I/II outbreaks are related to stress which can activate the dormant presence of intranuclear herpes virus and cause an outbreak ("cold sore").

One dental product that can increase sores in the mouth (apthous ulcers) is the chemical in tartar fighting toothpaste.

Floss gently but thoroughly. Plaque isn soft and only needs to be wiped off the teeth by the floss. Tarter (calculus) is plaque hardened with calcium. Aggressive flossing will not remove it properly. Isn has to be professionally derided. Please see your Dentist/Hygienist for a complete professional cleaning. Ask for tutorial on proper effective not harmful brushing and flossing.