Jennifer Love Hewitt is back to work, but a source on the set of her hit show The Ghost Whisperer says the actress is "having a tough time" in the wake of her breakup with fiancé Ross McCall.

"She is usually very bubbly, but she is more quiet and seems sad," the source says. "She usually hangs out with the crew between takes, but [Tuesday] she is only hanging out with her bodyguard."

The source adds that skipping work wasn't an option for the actress. "She is always so professional," says the source. "She would never even think to miss a day of work because of personal problems."

Hewitt returned to the set Jan. 5, the same day PEOPLE reported exclusively that she and McCall had called it quits over the holidays.

"They're both really sad about this," another source, who is close to the couple, said at the time. "Even their friends are surprised; they seemed really happy."

Hewitt, 29, got engaged to the Scottish actor, 32, in November 2007 after dating for two years.