Gypsy Rose Blanchard's marriage hit rocky waters after a post-prison phone call from her ex-fiancé -- something we now know her estranged husband finds awfully suspicious.

Here's the deal ... sources with direct knowledge tell TMZ that Gypsy's stepmother, Kristy, received a call from Gypsy's ex, Ken Urker, sometime in January -- right around the time the first season of her Lifetime show aired, and we're told he wanted to clear the air.

Our sources say Ken told Kristy he wanted to clarify the reason he'd broken things off with GRB a few years back. At the time, reports said he didn't like the newfound fame she was getting ... but we're told he explained he just wanted to give Gypsy space to grow.

During this chat, we're told Kristy asked Ken what his relationship status was, and he revealed he wasn't married -- but, also made it clear he wasn't trying to get back with Gypsy, or break up her marriage to Ryan Anderson.

Welp, Ryan isn't buying that ... sources tell us he does, in fact, think Ken made that call to Kristy in an attempt to reinsert himself into Gypsy's life.

Our sources say Ryan feels like most of the issues with Gypsy really started after Ken called ... and we're told he believes Ken was the trigger for the split from Gypsy, who's now filed to divorce the guy.

Whether or not Ken meant for that to happen ... we're told RA's pissed.

There's also this -- part of the reason Ryan believes all of this was calculated has to do with how quickly Gypsy cozied up with Ken after she separated from Ryan ... almost ASAP.

Remember, they spent a whole weekend in New Orleans together and even got matching tattoos. And, while they've publicly insisted there's no romance between them -- they've been spotted holding hands and looking very couple-y.

Bottom line, we're told things changed once Ken gave Gypsy's stepmom a ring.

With that said, Gypsy seems happy where things have landed -- so, at least for her, all's well that ends well?