2011 Chevy Silverado Engine Temp Fluctuates when cabin temp set high

I have a 2011 Chevy Silverado 2500 with a 6.0 L. I never noticed this before because I was living in the south and never really had to increase my cab temp too much. Well I just moved to northern MI. Shortly after I moved here I had to replace my oil pump. Just before winter. once I was done I noticed when I would drive or when truck was at idle if the temp setting in the truck was above 75 the engine temp would rise and come back down. Now the hotter I kept the inside temp setting “say 80-90 degrees” the fluctuation would be more often and stay hotter longer. I have flushed the coolant and added the desired amount. It still happens. I cant seem to find a leak but with weather conditions its been hard to check. Any kind of helpful information would be great.