Mash tun rise in temp?

Ok, so last night was my first all-grain attempt. After building a mash-tun from a 7 gallon ice chest, I went with a simple recipe. 8.2 pounds of grain. My strike water was 4 gallons and it maintained 156 degree's for 30 minutes then went up to 157 then 158 - I then opened the lid for about 3 minutes and it went back down to 156 where it stayed for the rest of the 60 minutes. How is a rise in temp posible? I have a wired thermometer ran to the interior of the mash tun so I don't have to open it up. Any thoughts?

Faulty thermometer?

Either faulty thermometer per annasdh, or there were hot and cool spots in the mash.

my tun used to increase in temp here in az in the summer out in the sun, its not hot enough for that yet, Im gonna say water getting into the digital probe assuming its a digital probe thermo? happened to mine but the other way, it went down like 8 degrees.