Swelling in the gums can be because of many reasons, such as:
a dental infection, traumatic injury
, a constant source of irritation in the mouth and many more. what has caused a swelling in your case must be ruled out after proper examination.
if you have a decayed tooth adjacent to the swelling most probably the cause is a periapical infection (infection below the tooth- caused by decay).
you must visit the dentist
at the earliest ans get an x ray
done. it will help the dentist to localize the cause of swelling and treat it accordingly. please do not delay as the condition can worsen. it seems you have already delayed it by a month.