I had a really big assignment due in for another course.
.. due in.. a phrasal verb = due, required on time
...for another course. - the assignment is for a different course, not the one you were thinking of. ?
I had (past tense) a really big (large) asignment (piece of work) ... due in ... (due, needed to be handed in soon) ... (the assignment is ) for another course.
Xd, why is your maths assignment late.
I had a really big assignment ... due in .... for another course. I was working on that.