If the coolant turns kind of milky white emulsion when the engine is running there is a good chance you have a leak between the lube system and the cooling system - quite likely at the oil cooler. The lube system operates at higher psi than the cooling system so it will push oil into the coolant. If you have to travel somewhere before repairs leave the rad resevoir cap loose so it can overflow a bit if it has to. These leaks are often very slow at first.
What is puzzling is why it appeared suddenly after a flush... the flush compound should not hurt the cooler....
I would be tempted to look at it like Artum Snowbird says ...
Would appreciate you posting what you found.
Lab test the coolant is a good plan.