2004 Infiniti G35 BLOWING COLD AIR

My car keeps blowing cold air once I accelerate it starts throwing some heat but once I let go of the accelerator it goes back to cold. Also the sign next to the speedometer that shows when the car is overheating goes all the way up once I turn on the heat but if I just accelerate it stays in the middle. Today I went and changed the thermostat and added antifreeze and its still the same the only difference is that now it doesn't throw heat at all its just cold air.