Why do I feel so insecure about my girlfriend's ex's?

Every time I think of my grilfriend and her ex's it churns me up inside...does anyone elso feel this way? Whenever she (occassionally) mentions one of them my stomach drops and I feel panicky, even though it's just a harmless comment. I hate talking about her past, even though I trust her completely. She is lovely and considerate and doesn't mention them to be hurtful, just in passing. I know she loves me very much, but for some reason I still feel a lot of anxiety around this whole subject. I have ended relationships in the past due to this issue becoming too overwhelming. Why do I feel like this and what can I do to stop (or at least minimise) it??