Covering a colleague whilst they are on maternity leave & not being able to go back to my old job

I have been told that my position is no longer available even though I took the covering position on the belief & guarantee that I would be returning back to my old position when my colleague returned to work. They are telling me I have to take a different job on a night shift rota even though my old job was an afternoon shift & I am a full time university student.  This is causing me a lot of stress & panick & I don't know what to do. She returns to work on the 4th of jan 2020