Sources with direct knowledge tell TMZ ... Nicola has retained a legal team and plans to go after the grooming company located in upstate New York that worked on her pup at a home she was staying at before Nala experienced a medical emergency that turned fatal.

We're told Nala had a check-up with her long-term vet prior to the trip in NY, and was deemed fit and healthy ... which is why her sudden passing has been so shocking to Nicola.

Sources say Nicola and her husband, Brooklyn Beckham, have been clients of the grooming company with no prior issues ... occasionally using this groomer while in town with family.

However, this time around, there were numerous red flags ... several of their dogs acted oddly following the groomer's services on June 14. We're told the dogs were dropped off one by one, as the groomer's van was stationed near the property. Sources say Nala was returned last and was in super bad shape.

As Nala was gasping for air with a blue tongue, Nicola rushed the chihuahua to an emergency vet ... where she died a few hours later. We're told vet records show Nala had fluid in her lungs, a crazy heart rate and neurological issues before passing.

Since Nala had no underlying health conditions, there was no reason for her to die following a simple grooming ... which is why Nicola is taking legal action. The actress' legal vendetta has nothing to do with money ... rather, she wants to prevent a similar tragedy.

Nicola -- a staunch animal rights activist -- wants answers about Nala's time with the groomer, but we're told there were no cameras in the grooming van ... raising questions about what went down inside.

Sources tell us the groomer is no longer cooperating with Nicola's legal team.