Brittany Cartwright's calling out her estranged husband on social media ... telling fans he's not donating money to cancer research -- and, telling Cameo to handle it.

The reality star posted a screengrab from a video on Jax Taylor's Cameo -- the platform where fans can buy personalized messages from celebs -- where he claims he's taking every dollar and donating it to a charity fighting cancer.

Welp, BC called out her former flame with a screen grab from the clip ... calling him "disgusting" for not really donating the cash -- and tagging Cameo in the post to let them know about the alleged fraud.

Sources close to the former couple tell TMZ ... fans of "Vanderpump Rules" know Jax has donated both his money and time to charity over the years -- but, Brittany's not sure he's ever donated to this one, and she got upset when she saw the vid.

Loyalty and honesty -- especially when it comes to charity -- are close to Brittany's heart, we're told ... so, this video really hurt her.

We've told you all about the couple's acrimonious split ... with Cartwright detailing the massive fight that ultimately ended their relationship.

The two have navigated their split -- with consideration for their 3-year-old son Cruz -- over the last few months ... and, JT recently went for in-patient mental health treatment. Basically, it's been difficult for everyone involved.

We've reached out to Jax's team for his response to Brittany's claim ... so far, no word back.