Although dehydration doesn’t cause diarrhea, the diarrhea certainly may cause dehydration and it’s very important you are drinking water.
It may appear to you as being “heat-induced” specifically, however, there are multiple reasons why anyone could be experiencing diarrhea. My advice would be to contact your physician’s office and schedule an appointment.
Until you’ve discussed this with a medical professional, I would also recommend not utilizing the sauna and certainly not increasing time at this juncture.
I hope it is nothing serious and this is all put behind you, (absolutely no pun intended). Have a nice weekend!