You have a continuity issue, likely with your mech switch. When the mod heats up, particularly in the switch assembly, it's a indication that you have a substantial resistance, causing a "
Joule heating" condition. If you're a decent enough conductor (sweaty hands)... you can get shocked.
If you've not disassembled and cleaned the switch assembly... do it. If you have a DMM, you can test the mod for resistance/conductivity... should be zero Ω. If you have an inline volt meter, you can test for voltage drop... another indicator or conductivity.
If you can't sort it out, or it seems uncorrectable, I would suggest raising the atomizer resistance a few tenths of an ohm at a time, until the switch cools down.