My parents read my messages and I don't know how

Hi,

My partner and I have been together for about four years but our relationship is still very closed and secret due to our society. My partner used to communicate with me through a fake account on Facebook so that we had minimal chances of getting caught. A few months ago my parents confronted me that they somehow knew that I was talking to this fake account (they thought it was real) and claimed to be able to see my messages on their phone. They told me that I should end whatever I'm doing now and I said I would. We no longer talk through that fake account but on our real accounts, but how they were able to read my messages then.

Here are some details:

- After sending every few messages I delete our conversation

- Talking to my parents about it is not an option

- I don't have access to my mother's phone, on which she claimed she can read my messages

- My Facebook is not connected to any mobile account

- I have changed passwords, 'logged out of all devices' many times- The things they knew couldn't be by accidentally leaving Facebook open because I deleted right away and am sure I wasn't logged on anywhere else.

- My phone sim is under my mother's name, not mine.


If anyone has suggestions as to how they could know, please leave a response. Thanks