Problem after changing apple ID and using find my iphone

I changed my Apple ID from my old email address to my new email address, after I changed my apple ID my iphone would randomly ask me to enter the password to my old apple ID, I would sign it out and then sign in under my new apple ID, now if I were to try to enter the password using the old apple ID, it would tell me that it was not an existing apple ID. I am not sure why it kept reverting back to the old apple ID that was no longer an existing apple ID but I did not have any issue signing my phone into the new apple ID when this would happen. However, my problem is that I lost my phone at Krogers Thursday night and when I tried to use the find my iphone, it says that my phone is offline. I have the internet on my phone and it was powered on but it could not find it and the last location it had listed was from five hours earlier. I don't know if there is anything I can do to resolve the apple ID issue so that it can be put in lost mode or wiped clean. I have tried to make a new apple ID with the old apple ID but it says that it is connected to an account already but if I view my account it is not listed on there anywhere and if I try to sign in with it or reset the password for it, it says it is not an existing apple ID. Has this happened to anyone else is there anything I can do to get connected to my iphone it was a 6plus