hello. welcome to HCM.
you have been correctly guided in that you need not worry for a stagnant swelling over the last 5 years. However, it is unlikely for a fall to cause a swelling. It is quite possible that you may have just noticed it after fall.
If you had an open type of wound, you may be having locallized reaction, which can also be like a painless swelling, though less likely.
To detect it, you can have a simple investigation like an FNAC, it will diagnose whether it is just a mass of fat (? lipoma
) or a locallized reaction to foreign body. You also have the option of getting it removed in case you have minor discomfort or for cosmetic reasons.