the antifreeze has to be going somewhere . check the exhaust for white smoke and check the passenger floorboards for signs of a leaking heater core. i loose a little from my overflow line . If you let it get too low , you can get air in the system that can get trapped in the heater core for a short time .after refilling , go through a few heat cycles and see if the problem persists that should burp any air out . if it heats up really quick , it could be the thermost is stuck closed , but usually you would start overheating if that happened .
Proud owner : 05 WK Ltd 5.7 HEMI
90 YJ Islander 4.2 Cam'd & Decked
94 ZJ Ltd 5.2
00 WJ 4.0 - new to me Aug/13