If non traumatic it could be either a swelling due to a local infection in the scalp area or maybe due to occipital lymph node enlargement again due to an infection in the surrounding region
Next Steps
look for signs like crusting, redness and palpitation but
it is best to get it examined in person at a nearby clinic
Do consult a GP in person
Health Tips
If it is painful you can use warm compress for both lymph node enlargement and local scalp infection for temporary relief