So strange but...?
Answers: It's probably your lymph nodes - you enjoy them in your groin, armpit and collar. These lymph nodes swell when you are fighting past its sell-by date bacteria - close to a cold - this is why your neck and arm pits hurt when you hold a nasty cold. Up your vitamins and carry more sleep.
There's also the possibility you pulled a muscle (there are muscles in there)...if you did, it'll fix itself - newly avoid overexerting it or doing strenuous activity that could irritate the muscles.
If you havent done verbs ups or push ups or some form of exercise that might make you sore beneath your arms, You should probably see a doctor.
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