Ok so my car is overheating why?

There aren't any leaks whatsoever. I did put on a new thermometer on like 2 weeks ago, today it completely overheated, so i opened the hood and there was a very small jet of steam coming from the thermostat housing, im almost sure i installed the thermometer correctly, when the engine was on i squeezed the return hose it was extremely hot and didnt feel like there was any pressure or any fluid running through, so would it be safe to assume the thermostat is faulty or installed incorrectly? the thermostat housing looks completely flush with the engine. could it be the radiator? also we have a 1999 mustang using 5w20 could a different oil help the temps or is that irrelevant?