Tori Spelling on NutriSystem

Tori Spelling was thrilled to welcome son Liam last month – but she wasn't so happy about the weight she put on during her pregnancy.

"I gained 40 lbs.," the Tori & Dean: Inn Love star, 33, tells Entertainment Tonight. "Ideally I'd like to get back to the weight I was at [pre-pregnancy], which was probably around 115."

To that end, she's signed up for the NutriSystem food-delivery program (which she likes because "It's not like you have to go and sit through a boring meeting," she tells ET) and has been spotted jogging with husband Dean McDermott, 41.

But being concerned about her weight is new to her, she says. "I've been lucky. I don't know if it was genetics or age. I would eat healthy at times and pig out at times, but I never had to go on a strict diet plan."

While she was pregnant, she had mixed feelings about her changing body. "I went back and forth," she says. "I would have some days where I would feel really proud of my tummy and walk in front of a mirror and be amazed by it ... and I'd have some days where I felt like a whale. I gained weight in places that I didn't know I could ever gain weight in."

Still, her weight worries haven't dampened her joy at being a new mom. "I'm on cloud nine 24 hours a day," she says.