Git keeps mentioning my old username even if I can't find a trace of it

remote: Permission to MYUSERNAME/project.git denied to OLD-USER-NAME

I have 2 GitHub users. One for personal use and one for work use. MYUSERNAME will be my personal one for this example and OLD-USER-NAME the work one. I've stopped using OLD-USER-NAME a while ago but I'm getting the above error when trying to push to a github https. I solved the problem by pushing directly to https://MYUSERNAME@github.com...

I'm trying to figure out why my OLD-USER-NAME was even mentioned and how git "knows about it".

Things I've tried:

1. ssh-add -l -> The agent has no identities.
2. I couldn't find any .ssh folder.
3. I don't have any ssh keys on both of my github accounts
4. git config --global --edit or git config --edit shows MYUSERNAME
5. git remote --verbose which gave me: origin https://github.com/MYUSERNAME/project.git (fetch) origin https://github.com/MYUSERNAME/project.git (push)
6. OLD-USER-NAME GitHub -> Settings -> Security History I can see my ip with a green dot next to it. Maybe it got something to do with it?

In addition to everything mentioned in the question and @gturri suggestions in the comments, the next step is to check whether git or the os are caching your credentials.

Check the keyring on your OSX / Linux / cygwin and also check if git config [--global] credential.helper has been set.