I feel some tingling in my left armpit occasionally from time to time. any causes?

Could just be: a pinched nerve. This is usually what causes tingling sensations, especially if they are intermittent. If it's super bothersome, you might want to seek out a Neurologist for further w/u and eval.

Determine the nerve: On the net, look up distribution of intercostobrachial nerve, medial cutaneous nerve of arm, and T2 distribution. See which correlates to the distribution of your sensation disturbance. Report your findings to your doctor. Since you are young and it only happens "from time to time" this may be benign and tests may be normal. However, if you can determine the nerve this can help investigation.