Please try not to blame yourself for what has happened. It is not something we usually see in guinea pigs and nobody can give you any answer as how and why the liver bleed has happened. It is just one of these crazy left field things that I lump under 'cosmic karma'. You cannot foresee them and therefore not brace for them; you just have to take them on the chin.
Concentrate on the happy times and the good life you have given your piggy. Guinea pigs don't measure their life with fixed expectations as to length of life or welfare standards. They measure their life in happy todays, good food and a cosy home, belonging to a group and feeling safe and welcome. If you have done that, then you have done all that has ever been required to. None of us has control over how long our piggies live and what they ultimately die from. Very often we are left with unanswerable questions. But this should not devalue or detract from the love and the many happy days we have shared and that are making the vast majority of the life.
HUGS