Aaron Hernandez's fiancee is concerned evidence from the ex-NFL's star's death could be discarded ... so she's suing the Dept. of Corrections to make sure they preserve every single thing that could possibly prove he didn't kill himself.

Shayanna Jenkins -- the mother of Aaron's 4-year-old daughter -- filed court docs Wednesday asking a superior court judge to prevent state officials from destroying video recordings, physical evidence and prison records connected to Hernandez.

And why?

Aaron's attorney, Jose Baez, has suggested Hernandez's death wasn't suicide -- it was murder -- and he vowed to run his own investigation to get to the truth.

It appears Jenkins is doing her part to help Baez's investigation.