I remember this exact pain with Lucas on the same side. When he latches and starts sucking, it burns like fire up my nipple, to my armpit and partway down my arm. The pain lasts about 10-15 seconds and the more he nurses the less it hurts. The nipple is also already agitated and hurts when it rubs against my bra. Ouch. I don't believe it's his latch because there is no pain whatsoever on the right side.
My initial thought is something getting clogged? I've tried feeding twice on that side before switching today even though it hurts. I figure getting it good and drained would be a good thing. It did bring done relief but the latch pain is still excrutiating.