Based on one of your other threads, it looks like you have the pump/UV in your sump.
More than likely this is your problem. You may want to turn off the UV for a few days to verify.
The UV does add a little heat to the water. The problem when it's in the sump is that you're going to pass some of the same water through the UV again and again which continually adds heat to the water.
To give you an idea, on my 450g I had a turbo twist 36 watt plumbed in/out of the sump. I experienced what you did and had about a 4 degree increase in temp. I got rid of that and instead bought 2 Aqua UV 57 Watt Twist sterilizers instead (equivalent to a single 114 watt) that would be effective plumbed in line with my main circulation pump at about 2000gph. Temperature increase doing this was only around a degree.