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Thanks for your question.
From your description it seems that you have developed a condition known as cystic acne where the infection moves deep under the skin.
The lump may be a part of infectious process occurring in cystic acne.
This is a treatable condition and you should see a doctor to get it treated.Your doctor may use some drugs in combination with antibiotics to treat the condition.These may include accutane, spironolactone, oral contraceptive pills, retinoid for local application etc...
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It it is increasing and decreasing in size it is not cancer.Most likely cystic acne as that may increase or decrease in size.
It may be some other benign cyst as well.It may also be a lymph node enlargement as a response to infection as that may also vary in size.
You need to have this seen by a doctor but do not worry about this being cancer.