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There are many possibilities for what this could be and the main way to figure this out will be to have your doctor examine you. Cancer is possible but less likely given several tings: generally cancer tends to be firm, hard, and would not usually be described as a water balloon. Water balloon-like feel is more consistent with a fluid filled sac called a cyst or a lipoma which is a fat-containing mass. The only way to tell is to have your doctor evaluate, but from your description, it sounds like cancer is low down on the list of concerns.
Please let me know if you have more questions.