i hope it is ok saying this, what you are describing sounds like it could be herpes,
a very large population has it, and it appears like a cold soar and generally breaks out when run down, or being stressed. besides getting those cold soars symptoms and being highly infectious its completely harmless,
most people get it from kissing a person where the its broken out or by drinking of a glass of a person where it is broken out.
i would be careful to kiss a client who appears to have a cold soar
i might would get checked out,
so far i know when it breaks out i think it has some yellowish filling and i think it stays a few days until it vanishes, and i think it can appear as a cold soar without breaking open and vanish after a few days again, and it is not infectious when it is not broken open.
i hope that helps and i hope i don't cause any fear if so i am sorry