hi this can be caused by sharp object like a crisp or sharp bit of your tooth catching it. I had this eating a crisp and it looks like a blister with bruised blood inside.
This will go down and if bursts will be tender and just use slightly salted water as mouth wash to keep it clean or baby teething gel worked for me.
I never felt me doin mine either until i felt the raised bubble in my mouth from my tongue.
hope this helps you