A gum infection can start gradually or quite suddenly. It is often poorly localized and nothing you do seems to make it better or worse. You may have noticed an increase in gum bleeding prior to, or during the infection. The gum often appears to be quite swollen and red. Try using Corsodyl mouthwash as a palliative (temporary) treatment, but bear in mind it is important you seek professional attention, because large amounts of the bone which hold your teeth in can be lost during one of these infections.
Smoking and some medical conditions can make you more at risk of gum infections - so it is important you make your dentist aware of anything.