Vaginismus is the involuntary contraction of the vaginal muscles at the time of penetration for people with vulva. While for some people penetration during sex (penis-vagina) is painful, insertion of a tampon is painful for others any kind of penetration like a tampon is also painful. Vaginismus is treatable and visiting your doctor will help your course of treatment. There are two types of vaginismus, primary, which is when any kind of penetration is painful and there has never been any penetration in the past. This is also called lifelong vaginismus. And secondary vaginismus is where a person has had penetration before but is currently experiencing difficulty. This is also called acquired vaginismus.
Sources:
https://www.nhs.uk/conditions/vaginismus/
https://www.healthline.com/health/vaginismus#treatment
https://my.clevelandclinic.org/health/diseases/15723-vaginismus
https://www.webmd.com/women/guide/vaginismus-causes-symptoms-treatments