It could be any of these 3 things:
-Your head gasket didnt seal and your getting exhaust gases into your cooling system causing it to bubble over constantly
-You have tons of air in your system from filling it back up with coolant
-Your o-ring on your overflow tank cap is pinched or shot. (Mine did this exact same thing, i replaced the cap with new o-ring and problem solved). TRY THIS FIRST, ITS CHEAP.
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