It was just a dense bud, that's all. This happens by genetics, or vaccuum sealed buds, or buds that have simply been compressed by accident.

There is a strain out there I have grown before called Rockbud. The name says it all. It is a 100% indica plant that grows no more than 3ft in height. The buds are hard as rock, and is a very potent Indica high. You could have come accross that or a cross of that.

There are just so many strain out there that it could be anything. Sounds like some good shit though.