Incoming facetime call from a contact turns out to be a stranger

I recently (two years ago) converted by whole life away from apple getting an android, then a pc and finally moving away from any apple software. Therefore, I have no access to apple tools such as facetime or imessage. However, recently, my girlfriend (who I met after I made the switch) recently received a facetime from my contact which she believed to be very strange as I have never called her from facetime (naturally). However, upon answering the call she was greeted by the face of a stranger (who was very clearly not me), she hung up immediately and called me which prompted me to research this occurrence. Not finding anything online I decided to turn to the apple community. She had my old american phone number which I have turned off and released back into the phone number pool and my current french phone number (we are studying in France). I told her to delete my old american number and restart facetime. Sure enough, my contact in recent calls was replaced by the american number (which as stated before is no longer linked to me). I have two theories: the first is that someone used my old phone number and somehow was able to link it to my contacts. The second is that apple horribly mismanages sensitive information and my contact information and god knows what else has been accessed from apple or the icloud. I'm just trying to find out what people think it could have been or if they have had any experiences with this. Also, any advice on what can be done to ensure my sensitive information is protected and my girlfriend no longer receives calls from strangers posing as me. Thanks for the help!