That's funny because I had the same issue last week when I did a coolant flush. I couldn't figure out the problem right away, took me a week...lol. I didn't tighten the block drain plug to recommended specs of 18lbs, probably because it a difficult location, had to use a 30 torx bit socket, and I re-used the gasket ring. Another issue was having an air pocket in the system, which will over heat until that air pocket is out.
I went to the BMW dealership and bought me a gasket ring for 38 cents and a vent screw (bleed screw) just in case for $3. I changed the coolant again, and flushed it one more time. Last time I spent $11.50 for each gallon of that orange shit from Prestone, used 2 gallons because I was losing water. I figure down the road I'm gonna have to replace that drain plug, so I bought that yellow shit from Prestone for $8 at Wal-Mart.
It's easy to get the air pocket out. This is what I did: With the car off, and the vent screw off, I started pouring a gallon of Antifreeze (pure) into the expansion tank. Then I grabbed a gallon of distilled water and started pouring that until the mixture came out the bleed screw area. A couple of bubbles came out then it poured out normally like it was spilling over. Tighten the bleed screw back on, be careful not to break it and use a big flat screw driver. I would say it took about half or 3/4 of the distilled water. Put the radiator cap back on, turned the vehicle on, turned the heat up, think it was 91, let it sit for about 10 minutes looking for leaks. Didn't overheat so I took it for a test drive, it worked out great.