MTX Help missing doses

Hi my hospital screwed up my script for my MTX so I have not taken an injection for three weeks.
I received my injections this morning now I need to know do I take it today? Then take it my normal day and go back to that day weekly ? Or do I take today and use this as my new weekly day ? I have asked my suppliers pharmacist but still waiting on an answer but I need the flare to stop any advice would be great