Credit Cards in Apple Pay removed

Wy are my credit cars I use in apple pay constantly being removed?! I just hooked up my phone to my Mac mini and iTunes and got a notification on the phone that the credit cards were removed. This is not the first time it's done it. This has happened even without connecting the phone to my Mac mini and iTunes. This is getting very tiring and extremely annoying, having to constantly add my cards!!!!!!!!!!!!! What can I do to correct this?