Paris Hilton's Mom Kathy Says She Was in 'Complete Shock' When She First Met Grandson Phoenix

Kathy Hilton remembers the first time she met her grandson Phoenix Barron Hilton Reum.

The Real Housewives of Beverly Hills star recalled how her daughter Paris Hilton surprised her with the news that she and her husband, Carter Reum, welcomed a baby boy together via surrogacy.

"I was in shock, I was in complete shock," Kathy told Extra in an interview published Friday. "We didn't have our Christmas together because she was working out of the country. [Paris] said, 'I have a surprise for you,' and it was my Christmas present. Then she waited a few minutes and goes, 'I have one more surprise for you.' "

"I thought maybe it's a puppy because my husband and I lost my dog," Kathy shared. "Anyway, she walks in, blanket falls down, and I just go, 'Who is this? What is this?' "

Paris, 42, told Rolling Stone that, after presenting her mom with a blue Chanel bag, "I was holding the baby on my shoulder with a blanket over him, and then I just sat down. She's like, 'What is that?' And I was like, 'A baby … meet your grandson.' She's like, 'Is this yours?' And she starts crying. She's like, 'Let me hold him. He's the most beautiful baby I've ever seen in my life. Oh, he's so beautiful.' She was just in tears."

Since then, Kathy has been "obsessed" with spending time with her grandson, as Paris told The Tonight Show Starring Jimmy Fallon earlier this week.

"[She's] obsessed with showing up all the time, unannounced," Paris joked. "I'm like, 'Mom, when did you get here?' 'Oh, I've been here for hours!' "

Kathy also told Extra of her grandson, "He is such a little angel," adding that he is "getting heavy and heavier every few days."

The Hilton matriarch previously told PEOPLE exclusively that she and husband Rick Hilton "are over the moon" after the birth of their grandson.

"Paris and Carter, we know you are going to be the most amazing parents," Kathy said in a statement to PEOPLE. "We are so thrilled for you both! Thank you for this perfect addition to our ever-growing, beautiful family."