I know where you are coming from I felt similar to how you have been feeling, which it is called retroactive jealousy. I managed to overcome the negative feelings I was getting but it took some work. I get the odd bouts now and again but the feeling is not that strong as they used to be. Its her past and thats where it should be left but easier said than done. Give it some time and you will see that it becomes easier to cope, you need to do something to keep you occupied. A good book I read called the Power of Now was a good self motivator.
My main problem was that even though my partner wasnt that promiscious its the fact I have had hardly much experience with the opposite sex and released it was more my issue.