Thanks for posting your query,
From your description it seems you might be having a lipoma
( a benign mass of fat) and usually when touched seems like a fluid filled sac. You will need to consult with your primary care
doctor who can examine closely to confirm the cause and decide on the best possible treatment. Other possibilities are a collected abscess
(firm circular non-fluctuant mass) and others etc.
Your swelling must be examined by a health care practitioner in order to determine the exact cause. It might require a surgical excision and later on laboratory analysis by a pathologist.
Hope this helps