Hay, first time mom anxiety, alam ko yan! Honestly, armpit tenderness is pretty common in early pregnancy dahil sa hormonal changes, at marami sa amin na-experience yan na walang malaking problema, pero since you're only 4+ weeks pa lang and concerned, better check with your OB na lang para peace of mind mo and para i-monitor din nila properly. Sana okay lang ito, but your doctor can give you the best reassurance!