It is reasonable to assume that both the hair and your breast floated in the water as you immersed, allowing the water to reach all parts of your body, and thus your
tevilah was
valid. (One should lean slightly forward during the
immersion to make sure the water can reach under the folds of the breasts.) Despite your suspicion that the hair got there before the immersion, it is also possible that the hair did not get there until after you immersed while you were getting dressed, in which case it had no effect on your
tevilah at all.
We respect a woman's right to learn from her experiences and trust her own judgment. On the other hand, in a situation of doubt, it is always best to clarify the halacha with a knowledgeable halachic authority.
Please feel free to get back to us with any further questions.
