Thank you for your question! And I am sorry to hear that you are in discomfort.
This "little tooth" in question is actually your lower right wisdom tooth.
The picture shows that the tooth is partially erupted from the gum and is still hidden under the gum flap.
When wisdom teeth begin to erupt out of gum as it is now, it's not unusual for infections to set in.
This is because the small size of the food gets trapped underneath the gum flap, and if it is left uncleaned, the infection of the gum overlying the wisdom tooth.
This condition is called pericoronitis.
https://en.wikipedia.org/wiki/Pericoronitis
If the pericoronitis infection spreads and gets worse, you may experience other symptoms such as a fever, afternoon throat or even swollen lymph nodes and general malaise.
I would strongly encourage you to see your trusted treatment, pros and cons and risks of having wisdom tooth removed, to potentially prevent future bouts of pericoronitis.
I hope this answer helps you and I wish you a speedy recovery.
Kind Regards,
Dr. Gerald Tan