The radiator becomes pressurised when hot so will be more prone to leaking along hair line cracks then. Could it be that there was some leak stop added previously and you drained it out when you did the flush? You could try adding this but it sounds like its not going to last much longer.
The BMW radiators are notorious for failing on the joint between the metal and plastic. I replaced mine with the Nissens one as they are supposedly better than OEM and cheaper too, so far so good. I also replaced all the hoses and thermostat at the same time as they were all getting dry and starting to crack.
Make sure you keep an eye on the temp gauge and as soon as it starts to climb stop and check it out. Do not continue to drive once the cooling system has failed as you risk warping the head.
Good luck with it...