I am very much thankful to your comment on my queries. Hope it will be very helpful to me.
This likely is a canker sore. I got one recently myself. They typically are very painful, but can go away by themselves. You want to avoid brushing the area with your toothbrush, stay away from acidic and spicy foods and don't irritate it with gum chewing. TYou might try to hold some plain yogurt on it (use a spoon if you need to) for a few minutes a couple of times a day- that can be soothing. If you've had it for more than three weeks time, see your dentist.