I seems that you are suffering from gingivitis which means swollen and inflamed gums. What you are complaining is a typical symptom of gingivitis.
It might be due to improper brushing or food impaction between the gums and the teeth either because of a gap or cavity.
You need to get the cleaning done for your gums and if at all any cavity present then get it fixed with cosmetic fillings..
Next Steps
Visit a dental surgeon for scaling i.e cleaning your teeth and gums with an ultrasonic machine and get yoursrlf scanned for any cavity prophylactically.
Health Tips
Brush twice a day daily without fail using vertical strokes and if possible floss at least twice a week to avoid such problems in future