May 18, 2015 02:45 PM

Sandra Bullock was blindsided on Mother’s Day!

The actress admits Monday on The Ellen DeGeneres Show that she spent the day crying thanks to her son Louis.

“It was all happy tears. I just cry at anything now,” the Minions actress, 50, says. “I didn’t [give birth to him], but I feel hormonally.”

The 5½-year-old — who insists on adding that extra half to his age, jokes Bullock — started her morning off with a sweet serenade.

“He sang me a song that he wrote … I didn’t have the heart to tell him he plagiarized,” she jokes. “[The song goes], ‘There was a mom I love so much and Sandy was her name-o.’ ”

Warner Bros.

Cue the water works. “I’m just like, ‘Ahhh!’ He played the harmonica in between and then he just kept looking at me,” PEOPLE’s World’s Most Beautiful star shares. “He’s all boy with a really tender heart.”

Bullock and Louis recently returned from his birth city of New Orleans, where they enjoyed Mardi Gras — much to host Ellen DeGeneres‘ horror.

“You took him to Mardi Gras? What kind of mother are you?” the Louisiana native jokes.

Although the proud mama playfully counters with, “I think a good one … if you’re gonna learn, you’re gonna learn,” DeGeneres quips, “Those are called breasts, son.”

“He’s like, ‘Why don’t they look like yours?’ ‘Well, I don’t have any,’ ” Bullock says with a laugh.

“No, oddly there were no breasts … I was too busy catching the beads. I was focused on the beads.”

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— Anya Leon

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