Q: Temperature not going past Cold on thermostat! on 1998 Mazda Protege

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
My car sat for 2 weeks at the mechanic to have the serpentine belt replaced (in well below zero temps the entire time). I get it back, turns over, runs hard for a few blocks as fluids get moving again. Got it home and there was no engine coolant in the reserve. I'm noticing that my thermostat is not moving past Cold anymore. Put in new coolant, level has stayed for 3 days. I run an average 10 miles one direction. Still blows somewhat warm air with heat on. Sorry, no codes. Thoughts?
(2) Answers
Change thermostat and coolant temp sensor also check heatercore for possible leaks or blockage.check your exhaust as well for any coolant getting through i think its more to do with the thermostat and the ect sensor. Also my friend in WI. If you need a tech to look at it i should be in la crosse in about 2 weeks