Inside cheeck or corner of gum problem,I'm not sure wich...help please?!

Inside cheeck or corner of gum problem,I'm not sure wich...help please?

Since yesterday,inside my mouth,bottom row at the left corner;I think I have a sore or something.It doesnt empede my chewing,eating&drinking,but when I clamp my teeth shut the little area in the corner of my mouth gets squeezed and it hurts.It feels like a sore,cuase when my jaw sqeezes it or a poke it with a brush or my finger,it hurts&sores(the pain 1-10,is probably a 4 or 6 at worse).But I'm not sure what it is.Only thing I can think of is the day before yesterday,I ate a lot of oranges,so maybe that acid from the oranges affected it or the other thing I thought was maybe becuase when I brush my teeth,a lot of time the hard part of the brush bangs against the bone&that's where the sore is pretty much,where the top and bottom meet and concave in the corner of the mouth@the edge of the last bottom left tooth.
Any body know what might be wrong?And how do I fix it?And is it something that will heal o it's own,or do I need to visit the dentist?

Thanks!:-)

Just for the time being buy a tube from a chemist shop called Bonjela, try it couple of times, it is quite soothing. If there is an injury it will give you comfort. Don't eat or drink anything for an hour at least. You will be fine. If it does not work, then definitely it is the gum. Put a clove there, you will have temporary relief, alternately you must consult your dentist.