Welcome to Health care magic.
1.The history you have given suggest most possible cause could be
lymph nodal enlargement ( painless lumps, long standing) followed by
lipomas ( fat content), vascular cause, glandular lesion.
2.At this moment if you want to evaluate the cause you need to get an
ultrasound examination of the neck done.
3.The ultrasound will help in assessing the sours of the lump and its nature ( solid , cystic and mixed) and where all it is extending.
4.If necessary a needle sample will be taken ( FNAC - fine needle aspiration cytology). That will confirm the diagnosis.
5.Not to worry regarding cancer and all - to young to think of that. but its on your body and bothering you so find out what it is, is you able to do. Good luck.
Hope it helps you. Wish you a good health.
Anything to ask ? do not hesitate. Thank you.