Understanding Li-ion Battery(ICR18650-26H)
It sounds like the battery has a PTC protection device, the PTC acts as a self-resetting circuit breaker which limits the maximum current that can be drawn from the battery to a safe(er) level. If you want to measure the temperature you'll need a separate thermistor. Temperature measuring thermistors are NTC devices - their resistance goes down as they get hotter, PTCs are the opposite, their resistance goes up as they heat up, this is why they make good protection devices, if you try and draw more current, it heats up more driving it's resistance up, limiting the current - the PTC will be in series with the cell.