First of all, relax. Most kids/babies go through periods of having different colored stands in their hair.
My daughter had blond, very light hair and had one strand of black hair that came through.
It's probably not the shampoo. I was a hair dresser for several year and never heard of shampoo removing natural hair color (unless it was made to do that specifically).
My mother in law also got a grey streak as a kid and so did my aunt. Both have lovely hair to this day.
If you are overly concerned with it, ask your doctor when you take her for a check up.... and try not to cut it... not that it will cause more to grow, but it just will continue to grow and you'll be forever cutting it to get rid of it... it's hardly worth it.
And please don't make a big deal about it to your daughter....if you don't let it bother you, it won't bother her either.