Brief Answer:
Most probably an infected sebaceous cyst / boil

Detailed Answer:
Hi.
Thanks for your query.

To recapitulate: young 25 / Female - firm pea sized lump - right armpit - tender - similar history before too -
Wants to know the possible causes: which are:
If the lump is easily palpable and just under the skin- this is most probably an infected sebaceous cyst.
The other less probable cause is a lymph node. Lymph nodes are deep-seated and not easily palpable. There may be a possibility of a focus of infection in the drainage area like the same side upper limb or breast.

Cancer are very rare at your age, the nodes are painless to start with, so nothing much to worry about it at the moment.

I would advise you to get a proper antibiotic and anti-inflammatory like Ibuprofen medicines for 5 to 10 days under a Doctor's examination, supervision and prescription.
This will resolve.
If you see no responses consult a Surgeon to get proper examination and diagnosis and treatment.

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Brief Answer:
Head may not be seen in every case.

Detailed Answer:
Thanks for your feedback about slight bulge and a bit of redness. No head or anything like a boil or pimple - may not be seen or visible in every case particularly in the armpit and when infected.

Giant pimple : If the cyst enlarges over the time without natural resolution or abscess formation, then only it can appear as a giant pimple.

If this is attached to the skin, this is likely to be a boil. All the cysts are small to start with and if get infected, the appearance of both will be similar and the initial treatment remains same.

I hope this clarifies your point.

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Brief Answer:
Yes, it can....

Detailed Answer:
Yes, the cyst can get resolved on their own. Healing is the body's own system, we just assist by the medicines and / or Surgery...

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