Probably you want to say that there is swelling. You have not provided any more details like color and which side it is.
There can be many problems which is the cause of it. It can be due to abscess (pus) in relation to any tooth, it can be due to blockage of any salivary duct or opening, or it can be due to trauma from any sharp tooth cusp and there are various other several reasons.
Next Steps
Visit a Good nearby dentist and get it diagnosed as soon as possible.
Health Tips
Don't indulge in any self medication and don't rinse with warm saline water.