are they friends and do I have any reason to be worried

[Rich in Love]

I was married for 17 years and am going through the divorce now. I am over my ex-wife and have no bad feelings there other than some scars from the affair she two years prior to the divorce. I meet my new girlfriend (Amy) and at the begining of our relationship she meet up with her ex b/f from 5 years ago. At that meeting they discussed getting back together. She told me she had contemplated getting back together with him. I felt unsure about our relationship but after several fights about this issue, I began to see the light that she was with me and not with him. So 6 months into our relationship we were going on vacation. She called her ex for his birthday the day before we left. She told him "she" was going to Costa Rica on vacation. When she told me about this I asked her if she told him that she was going with me? She told me that he does not like to hear about her personal life and that he does not even know she is in relationship. I felt like maybe he was not done with their relationship. Amy told me he is living with another woman but Amy felt he would leave his girlfriend to get back with Amy. So I thought this was more evidence that his "friendship" was more than that. I do not think Amy feels the same about him but she feels I am being insecure and suspicious about their relationship. When ever I bring up the issue we fight and nothing get resolved. Am I being jealous or is she fooling herself?