As you describe possible causes may include Perineal cyst/Abscess,Epidermoid cyst, lipofibroma.
If it is progressive in size over the time and becomes painful or tender; you may need to undergo a clinical examination by your regular doctor and treat it properly.
As it is fluid filled so further incision and drainage may be required.
Antibiotics may be given as well.
In my opinion,you shouldn't ignore it as such and don't leave it untreated.
I suggest you to consult your doctor for further evaluation and management.
I hope this helps.
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