Reese Witherspoon on October 24, 2012 in Beverly Hills, California. Credit: XPOSUREPHOTOS.COM

Hot mama Reese!

Reese Witherspoon just welcomed son Tennessee one month ago, and if the actress is sleep-deprived and exhausted, she sure didn't show it during an outing Oct. 24 in Beverly Hills.

Showing off her post-baby body in black skinny jeans, a white blouse and blazer, mom of three Witherspoon, 36, was all smiles as she ran errands and had lunch. Although she gave birth to son Tennessee James just one month ago (on Sept. 27), the Oscar winner has been out and about looking svelte and serene.

Named after his mother's home state, the boy's father is Witherspoon's husband of a year and a half, Jim Toth. The infant joins siblings Ava, 13, and Deacon, her children with ex-husband Ryan Phillippe. "Deacon is excited to have a brother, and Ava can't wait to babysit," an insider tells Us Weekly of the baby's older sibs.

And although Witherspoon's CAA agent husband was a bit "nervous" about officially becoming a first-time dad, he's "very excited his son is finally here" confirms a second source. "He's been there for Reese every step of the way."