Is the use of magnesium causing me itchy arm pits ?

It is doubtful that the magnesium is causing itchy armpits. Many people use magnesium oil as a deodorant under their arms with no problem. It could be possible that since you have increased your use of magnesium oil, more toxins are being released from your body and are coming out in the armpit area. Either cut back on the amount used and see if it stops, or bare with it for a week or so as some skin irritations do go away after you become accustomed to the oil. If irritation is too intense stop use of magnesium oil and apply baking soda to the area instead.