Could be, but you're probably overloading the tool, which makes the battery protect itself.
Continuous overloading causes permanent damage. If you're forcing it in dense material, the tool draws too much current, the battery overheats, and it shuts down. Ease up on the pressure. Let the bit do the work. If the tool stalls, reverse the motor to free the bit instead of pushing harder.
If that didn't work, check the battery contacts on the tool and pack. Clean off any dirt or debris. Try a different M18 battery. If the new one works fine, your old pack might be damaged. Don't use damaged batteries—they can be unpredictable and pose a fire risk.
Heads up: Never expose batteries to fire or temps above 265°F (130°C). Charge only with the specified Milwaukee charger.