sonic03blue

So my CEL threw a p0125-insufficient coolant temperature. I have noticed my car has been driving pretty cold lately and came to conclusion that it might be my thermostat stuck open. I also changed my coolant out about three weeks ago and may have messed up the mixture(pretty sure I didn't though..the hoses feel like coolant is at regular temp). It has been cold around here lately and I have been driving with my heater on..don't know if this could lead to anything with insufficient temp? Any help with a possible solution or diagnosis would be great! Thank you.