Wendy Williams' guardianship is being investigated by Adult Protective Services in New York City, and TMZ has obtained the call for help that sparked the whole thing.

A caregiver not associated with the guardianship -- Ginalisa Monterroso -- sent a letter recently to APS formally requesting a probe into what she saw as "troubling circumstances" surrounding Wendy's treatment.

In the letter, obtained by TMZ, Ginalisa raises issues about Wendy's restrictions at a New York assisted-living facility, where she's confined to the 5th-floor memory unit and not allowed to leave without approval from staff.

Monterroso says the circumstances raise "significant concerns regarding her legal rights, personal autonomy, and overall well-being, which warrant immediate attention."

She says Wendy is NOT incapacitated, as her guardian claims, and says the discrepancy "raises serious concerns about the integrity and motivations of the guardian, suggesting that they may be misrepresenting Ms. Williams's actual mental and emotional state to the courts."

Monterroso adds ... "Such misrepresentation could be contributing to her unjust confinement in an environment that restricts her freedom and social interactions."

The letter mentions our recent TMZ documentary, "Saving Wendy," where WW breaks down her life in isolation.

Monterroso says Wendy's confinement to the memory unit is bad for her mental health ... and she worries it also violates her legal rights.

APS is taking her concerns seriously ... as we first reported, there are now two separate investigations into Wendy's guardianship -- one by APS and another by the NYPD.