'The Walking Dead' Star Kelley Mack's Cancer Suddenly Relapsed Before Death

Kelley Mack was feeling much better amid her cancer battle months ago ... but everything suddenly took a turn for the worse right before her death, TMZ has learned.

A family source tells us ... in February, "The Walking Dead'" star underwent treatment for glioma -- which is cancer of the nervous system. After her treatment, the source says Kelley experienced a "nice recovery."

The source says Kelley's cancer then relapsed out of nowhere, spreading through her body in recent weeks ... adding she had a "quick end."

TMZ learned Kelley spent an "immense" amount of time with her family and friends leading up to her passing ... and she was even working on voiceover jobs a month ago.

A rep for Kelly tells TMZ her death was a "total" surprise because she had been auditioning for acting roles as recently as early June ... auditioning for roles as someone with mobility issues, because she was in a wheelchair.

She told her team she needed to take a break from auditioning in early July, and the rep adds, "no one expected" her death.

Kelley currently has a commercial for Rakuten, an e-commerce site, airing -- which she filmed with her mom, Kristen, in April. She also has a movie titled "Universal" making its way through the film festivals. She shot the film last year, according to our sources.

Kelley's family announced her death Tuesday, revealing she took her last breath "peacefully" on August 2 with her mom and aunt Karen by her side.

The "9-1-1" actress announced in January she had been diagnosed with the rare type of astrocytoma cancer after experiencing pain in her lower back and legs last fall.

She is survived by her mother and father, Kristen and Lindsay Klebenow, sister Katherine, brother Parker, grandparents Lois and Larry Klebenow, her aunt Karen, and her boyfriend, Logan Lanier.

Her family announced a celebration of life will be held in Glendale, Ohio on August 16, and they plan to have one in Los Angeles as well.

Kelley was 33.