2002 Chevy Blazer Temperature Gauge

I noticed today that my temperature gauge doesn't seem to be getting hot enough. It's usually at about 1/2 way on the thermostat, but today it was about 1/4 of the way up. I did have heat, but it didn't seem as hot as usual. Also, the service engine light came on yesterday. I checked the anti-freeze level from the overflow and it does seem low. And, I had a radiator flush in January.