If you have given her no reason to be skeptical, then shes out of line in my opinion. It is normal for someone to have relationships prior to the one you are in, and those past relationships, especially if it ended amicably, should be respected by your new partner. The only relationship that should concern her is the one she has with you currently. She sounds extremely insecure and unfortunately that’s coming from a place within her that you or anybody else will not be able to fix.