Deavan's planning to file suit against Jenelle for defamation and cyberstalking ... according to her publicist, Lindsay Feldman of BrandBomb Marketing. As for how their relationship got this nasty ... it's a bit complicated, but the gist is a few months ago they teamed up for a podcast called "Girl S#!t" -- along with Gabbie Egan, and when it didn't work out, the mudslinging began.

Deavan accused Jenelle and Gabbie of bullying and ganging up on her, and Jenelle accused Deavan of spreading lies.

It all came to a head recently though, with Jenelle posting a 44-minute video explaining her side -- but also dropping an allegation that after a weekend with Deavan, Jenelle's supply of anxiety meds for her esophagus problems turned up missing. Deavan later alleged the medication Jenelle seemingly accused her of taking was Xanax.

Deavan vehemently denies that, and it's one of the main reasons she's threatening to sue Jenelle.

While she's at it, we're told she's also considering legal action against Gabbie for allegedly violating an NDA related to the podcast.