Its normal when the engine is working harder for the temp to increase a little - how high has the guage indicated and is it in danger of overheating? Is the fan coming on when it should?
I would flush the coolant if that has not been done and make sure the new coolant is within specs for concentration. Use distilled water only for mixing with the correct coolant. It should be done every 2 to 3 years anyways, regardless of mileage, so you are due.
I rarely recommend additives, but one I do use is called "Water Wetter" by Red Line, added when you change coolant, as per the bottle's instructions. Its sold in 12oz bottles at any autoparts store and is supposed to reduce high temperatures.