there is no such thing as a sending unit unless they are refering to a water pump. that or your coolant elbows could be leaking but it is unlikely with low mileage. who knows double check
there is whats called a reverse flush on your heater core. sometimes they get a little clogged. theres 2 hoses. take them both off and use a hose to push water through both ways gently until you get an even flow in both directions. you should notice that the flow starts off a little slow and then gets fast. if it does then it was slightly clogged. and yes at a 100k miles you want to put some fresh anti freeze in there so just do a complete flush and you should be good to go.
if that doesnt work then you have an issue with your "blend actuator" i think is what its called where you control the temperature of your air coming through from hot to cold.
