I have a 2014 Chevy malibu that was running fine. It is overdue for an oil change. My car typically runs between the first eight and quarter mark even in not weather, running the air and in traffic. A couple of weeks ago I noticed the temp gauge was running about a quarter mark hotter than normal. Today, it went from the almost half mark to another eight and started to climb towards 3/4 mark. Turned on the heater full blast/maximum heat and it bounced all the way back to C.
Similar symptoms happened in my old Focus. It turned out to be a coolant leak. Check the coolant levels, and if they're low, refill and watch for liquid under the car after leaving it parked for an hour or so.
Low coolant level. If you don't have enough coolant, the radiator can't pass hot coolant through it to cool it off. Check your overflow tank, make sure it has coolant in it.
Clogged radiator fins. The fins help to create surface area so the heat can go from the radiator to the air. If the fins get clogged, it isn't as efficient. Clean with hose and nozzle from engine side out. Be careful as the fins may be brittle.
Bad radiator cap. If it opens too soon, it will cause the fluid to out out through the hose to the overflow tank. If leaky, may be allowing air into the system. Replace.
Bad thermostat. If it opens too little (stuck) it won't allow full flow of hot coolant to radiator. Replace.
Failing water pump (or bad/loose belt to it). If the water pump doesn't turn fast enough then the total coolant flow is affected - not enough coolant passes through the radiator. Test belt, see if loose or going bad, replace. Test water pump and replace if necessary.