The thought that she might never sing again crossed her mind frequently and being forbidden from talking, even just for two weeks following the same surgery that pop singer Adele underwent, left her feeling vulnerable and completely disorientated.
"I'm a very vocal, loud person by nature," says the singer whose ability to project lyrics in a seemingly endless variety of languages, and sound like a native in each of them, has given the beyond-stylish Pink Martini a remarkably cosmopolitan output. "And I'd never had any throat problems before. Except one time in Paris when I had a cold and was worried I might have done some damage until a doctor there told me my vocal cords were perfect. But I'd obviously developed a polyp since then and I must have popped it, so I had to have surgery and rest my voice."
When you've been onstage since the age of four – Forbes made her pre-school acting debut alongside Al Pacino – and you've been in the habit of shrieking at after-show parties and any other social occasion, complete vocal rest is, she says, so stressful.
"I couldn't even tell my two-year-old son not to fall down the stairs," she says with mirth, tempered with relief that she didn't actually have to do this. "I couldn't write songs because I need to sing as I write, and I started asking myself, have I been singing incorrectly all along? Will I ever sing again and when? The upshot is that now I take extra care with my voice. My surgeon examined my vocal cords after the operation and said he couldn't tell which one he'd operated on. So that's good news and I've been singing effortlessly since then. In fact, I can do things now I couldn't do before."
Although she didn't go on to become a child star actor after her early experience with Pacino, Forbes does seem to have been born to be on the stage. Her parents worked in television and theatre – her mother was a producer and her father a lighting designer – and as well as the then up-and-coming Pacino, they gave early stage opportunities to Stockard Channing, Paul Benedict and Paul Guilfoyle, who currently stars in CSI.
Forbes' interests lay more in music and languages. Half-Scottish (her great, great, great grandfather emigrated from Aberdeenshire to Boston and bought an island there that the large extended family still use as a summer retreat) and half-French, she developed her talent for accents from listening to her French grandfather's "very cool" way of speaking. She worked in Italy as a nanny soon after leaving school and still hopes one day to put her Italian to use in singing opera. She has also sung in Spanish, Greek, Ukranian, Portuguese, Japanese and Chinese and does a fine line in convincingly Cumbernauld-sounding dialogue from Gregory's Girl, her favourite film as a child. "Being a singer, I think, helps me to hear things clearly and makes it easier to be a mimic," she says.
"But if you want to know the real secret for sounding native in any language, you need to pretend you're drunk. That's what I do and I think that's very important. Well it seems to work for me."
Towards the end of her enforced absence from Pink Martini, Forbes began working on the soundtrack for a film her movie-producer sister is making about their family. She's continuing to write songs as she travels with the band again but instead of taking her guitar on the road, she has borrowed her son's ukulele, which fits much more easily into her luggage.
"It's just a cheap one but I hadn't realised what a nice instrument the ukulele is," she says. "It's the perfect accompaniment to the voice. I've borrowed a guitar a couple of times since we came out on tour but it feels so stiff compared to the uke and when you're strumming quietly to yourself late at night, the uke feels so much more personal.
"I'm probably playing the chords all wrong because I'm used to the guitar but I might get a good ukulele and learn to play it properly.
"Will I bring it onstage with the band in Edinburgh? You never know, I just might. It's certainly been a wonderful discovery."
Pink Martini play the Usher Hall, Edinburgh, on May 7.