Hello, As Of Recently My Tampons Wont Stay In And

Hello, as of recently my tampons wont stay in and wont go in as far as they used to. I would always use super plus ob tampons without applicators and never had any problems. My flow has decreased a lot, and I've tried using smaller tampons with and without applicator, but they just wont stay in. It has come to the point where I can feel it inside me when I'm sitting down, which is quite uncomfortable. I have noticed that the inside of my vagina feels thicker. (I'm 19 years old, have been getting my period for 6 years now and I've never had sex)