First of all, I want to say that as I was writing the title of this post, the page suggested an already solved question whose answer is World Without End by Gwynplaine MacIntyre, but despite the striking similarities, it's almost certainly not it, because that short story was published in 2010 and I read this one back in the nineties or the early 2000s. (Now, it's possible that MacIntyre's story is actually older than the publication date I found, but the details I remember are still different; perhaps this short story inspired his?)
So, this short story has a woman who has a capsule made of nanobots, designed to preserve her life in a variety of external conditions. I thinks she travels across space so it makes sense that the capsule must adapt itself to different environments.
The capsule is successful in preserving the woman's life to the point that when it becomes necessary, it melts with the surrounding landscape in order to get materials and energy to keep itself functioning.
The woman is kept alive until the Sun dies, or even beyond that.
EDIT: I live in Argentina. I read this story in a digital SF magazine called Axxón, which started publishing around 1989 (in diskettes!). The magazine is in Spanish. I wish I could check the old issues but they don't run anymore on modern machines.