My ex-husband has re-married, and I'm happy for him. I must admit, that if he and his new wife had had a child, it would have been very painful for me--because I've never been able to have any.
All that to say, I think it's natural, really. I'm sorry for the pain you are feeling, the grief you are re-experiencing. Please take good care and know that in time, the pain will lessen!