It could be that the tampon is to big for you (try a smaller size), and we have to remember that the female body is not a one size fits all, therefore, we are all made slightly different than the next and what may be straight in one female's inner body may be curved in another, but still normal.

If you are having pain during the insertion of a tampon and during sex please see your GYN for a check up as some medical issues can cause vaginal pain.