I wrote, previously, however, I felt I did a shoddy job with getting my thoughts across, so I deleted my post, but wanted to mention that, if she doesn't appear to need or want an explanation, I might refrain from doing so. Depending on the subject matter/ what you are seeking reassurance of, perhaps, you could mention that, at times, particularly, while experiencing more anxiety than usual, you can experience a heightened sense of uncertainty, in general, and reassurance can relieve some of the uncertainty you feel. I would only bring this up with her, if you trust that she would be understanding/ empathetic towards you. Otherwise, you could wind up feeling even more anxious than you had, prior to explaining.
I, also, believe that it can be of benefit, to develop the ability to seek reassurance from within oneself, because there will be situations, where acquiring it from outside of ourselves, will not be possible. It is simply, a good skill to possess, and, can be a healthier practice, than becoming dependant on outside sources to calm us, depending on the situation, of course. There is nothing wrong with wanting, needing, or asking for reassurance, however. It is human nature, and if your friend cares for you, she will understand, as opposed to perceiving you as being too needy.
