I once carried a log that weighed about double my body weight and carried it 100ft. My back has never been the same, I really don't recommend it unless you know what you're doing.
And you are carrying in your hands, not balanced on your head-- which totally alters the stress on the spine.
So yes, cograts on your feat, that is a pretty cool level to hit, but it doesn't counter the basic advice of "grow into it" instead of "max from day one"
Also, you did this "once". If the accident rate is 1:100, doing it once without getting hurt is not very surprising.