New owner question about battery behavior

Howdy. My new car was behaving differently yesterday. I drove my same route to and from work & daycare, but it fought me all day. I was hardly able to stealth at all. Whenever I would try, it would pull power from the battery, instead, even when the battery was charged way up. Even when I was coasting down the big intracoastal bridge of my route, it kept trying to come on. I know I've onlly had the car for two weeks, but before yesterday, the battery never pushed when I was coasting. Two days ago (again - same route), my mpg mileage was 53.6, but it dropped down to 52 yesterday. Not complaining - 52 is fabulous - but I am concerned about the car's change in behavior. Why would it be suddenly different? Is it something I have done? FYI - weather has been pretty steady, with moderate daily temps ranging between 60-70. I have put just 520 miles on it.

Thanks for any info you can provide.