Temp Gauge not reading, Help!!!!!

Its a really strange thing, I turn the ignition to on and my temp gauge would go up. Then after maybe a minute it drops all the way down. Has anyone encounter this issue. I have a 03 Yukon XL 5.3L. Could it be my cluster or coolant temp sensor?? I just had it in the shop for a EFI/TB cleaning when I got it back my temp gauge doesn't work, dont know if thier connected. Please let me know if anyone out there have any good advise, Thanks