1993 Toyota Pickup engine overheating

I recently replaced the headgasket on my 93 toyota pickup 4 cyl. manual. 2.4 fuel injected. replaced thermostat everything except water pump. I ran it shortly a couple times and everything seemed ok..till i checked it last night i noticed it a quart low on oil. I added oil and went for a 20 mile drive. it started overheating again so i shut it down. when i looked under the hood i noticed the coolant overflow was filled with oil sludge. would this be a water pump leak or a poorly torqued head bolt? Im really at a loss for what it could be now. and also when I did the head gasket i bought a new rebuilt head. so i think the head is not worped or cracked. any advice would be helpful thanks.
Zac