Most likely the tampon is "hitting" your cervix my dear.
When I Put A Tampon In, It Feels Like It Is Hitting Something Hard. What Is It Hitting?
When you insert a tampon you are hitting the tip of your cervix.
Because of the fact that you have a G spot and that is why it does that
If you are having trouble it only means that your muscles are a bit contracted. Make sure that there is enough lubrication before you insert the tampon and you should be fine.