Nissan 200sx GA16DE heat doesnt work after coolant change? heater core?

[TitoD420]

i have a 97 nissan 1.6L about 120k miles. I did a coolant flush around july this summer in 2008. I live in denver and two nights ago it was about 17 degrees here Fahrenheit and my defrost or heat did not work at all; i had to pull off to the side of the highway and my blower heat from my vents did not increase at all. For the past couple of days it seems like when I turn my heat on the engine temp gauge IMMEDIATELY drops. Weird. I took a year of basic auto mech and this would tell me its either something to do with my thermostat or heater core? otherwise does it have to do with my coolant level/ or possible air bubbles in it? Any advice/help/recommendations will help~ thanks!

[RED_DET]

When you did the flush, you should have relieved the system of air to insure proper heating. I'm sure this is the case. If it were your thermostat, it will show on the gauge that it isn't heating up to normal.