Engine temperature behavior

I've got a 99 nissan altima with 116k. I recently replaced the thermostat and flushed the cooling system. It is extremely cold here in the western suburbs of chicago tonight. I was out driving my car about an hour ago. I went from my friend's house to the gas station. My car had warmed up to normal operating temperature which is about halfway up the temperature gauge. After I filled up I started the drive home. I was getting colder so I turned the heater on full blast. When I did this my temperature gauge dropped from the normal halfway up the scale to only a quarter way up the scale. This seemed kind of strange so I turned the heater back down and it went back up to its normal position on the scale. Is this normal (it is my first car that has a temperature gauge so I have never have seen this before), and could the engine have been harmed by this sudden cool down and warm up? Thanks!

No harm done. Keep an eye on it. It may be perfectly normal, a sticking thermostat, or a failing temperature sender for the gauge. If it overheats or doesn't get up to temp, post the symptoms and we'll troubleshoot from there, but a short trip (less than 10-15 minutes) might not be an accurate test since it doesn't get fully up to temperature.

its prolly just normal, by turning the heater on you are removing more heat from the coolant system via the heater core. this can be a handy thing to know if your car ever starts to overheat

thanks for the replies. I'm going to be driving a lot tonight so i'll post back if anything strange happens