Please help my truck overheated?

my truck is a 98 chevy 2500 6.5 turbo diesel.okay so a few weeks ago i put coolant in my truck, and the low coolant light was still on. i didnt worry about it cuz it wasnt overheating. so i didnt mind it, the temp never even goes to half, only a lil over a quarter. the other day the check guages light came on, the temp was betweem 210 and 260, i pulled ovee and shut if off, all a sudden it atarted steamin from under the hoos. i popped it an let it air out. i put mor coolant in it and i tried to get it home. shortly it overheated again. so i had to tow it home.what is thr problem do you think? i think it may be the water pump or the headgasket. hopefully not cuz i have to plow