2010 Ford Fusion Engine Overhearing/Burning Coolant smell

I own a 2010 Ford Fusion Hybrid. Today, I was in the process of driving back to the office from a local town about thirty minutes away when the car alerted me that the engine was overheating. The digital engine temp indicator went completely red. This startled me, as it has never happened before. The car continued to run, although I noted with acceleration above 60 mph, it seemed to having a difficult time of it, so I drove slowly all the way back to the office. Eventually, the temperature went down, but I noted the engine temp level fluctuated from "normal" to "high" (but not turning red) the entire trip. The check engine light also came on during this time- it was not on prior to the engine temp indicator turning red.

When I got back to the office, I looked under the hood and noted a sweet smell that must be coolant that had a bit of "warm" smell to it- just short of a burning smell. I checked the coolant and I could see it- but it seemed low. I let the car sit for awhile and then re-started it. The check engine light turned on and I continue to get the sweet slightly burning smell of coolant. I drove it to the dealership, where it is now, and during the car ride, the engine temp indicator turned red again, but then returned to "normal", with rapid fluctuations between normal and high (but, again, not returning to "red") as the car increased speed. Upon arrival at the dealership, the warm sweet smell of coolant was again very noticeable. The Ford dealership will not be able to look at this until tomorrow, so I'm just curious as to what people think this is? A head gasket? I've never experienced this problem before, and the car gave no sign before now that anything was wrong with the coolant or the engine temp. The car receives routine maintenance, and no one has ever indicated there was a problem with coolant leakage. What's going on here? If it is a head gasket, what is a reasonable cost for repair for that? Thanks in advance.