The area of my gums right behind my last tooth on one side of my mouth on the lower jaw is paining a lot! why? what should i do?

INFECTION, get seen: You should get seen for treatment (may need antibiotics for an abcess) . Later, your dentist may take an xray to see the extent of the infection/inflammation. Hope you are better soon!

Impacted wisdom: How old are you? Have your wisdom teeth erupted? An infection in the area is quite possible, however you may have an impacted wisdom tooth that is causing inflammation and pain. See your dentist for an examination.

Call your dentist!: Can't diagnose or treat your problem over the internet, so you need to call a local doctor for help. Feel better soon.

Pericoronitis: You are exactly the age when we expect eruption of the wisdom tooth into the oral cavity. It is so far back in the mouth that hygiene is harder to maintain. Usually what we see is: a third molar partially covered with gum tissue, acting as a pocket for food and debris to collect:this allows what we call pericoronitis to occur (peri=around, corona=crown, itis =inflammation). Several episodes=remove.