Have checked your oil dipstick and oil cap? If it looks like chocolate milk it's either a blown head gasket or cracked head. Also, not sure if you know or not but you have to bleed the system when flushing out and replacing the coolant. Google year.make model and engine procedure for bleeding cooling system
Most common problem is air trapped in the system. You could also have a shop pressure test the system to see if there's an internal leak.
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