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The lump in te armpit is likely to be a lymphnode that is enlarging due to the local itchy rash on the chest and back,
The itchy rash is likely allergic, but the cause of rash needs to be looked in to,
Using benadryl orally, should help along with local application of 1 % hydrocortisone cream available OTC,
For persistent symptoms, you must be evaluated by your pediatrician though,
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