Why does my ac compressor come on when I turn on my switch for ventilation fan,fan on thermostat is set for auto and is in heat mode, could I have wired my ac wrong? Never had this happen before,even though the ac is only 1 year old

Yes I would say its wired wrong. Give more info- Type of system, if you changed the t-stat what wires did you use?

The sticky at the top of the forum has a complete description of how to wire a thermostat.