Bronwen Newbury, 27, said she saw the pink paint on the side of her Kia Ceed when she walked into the staff car park of an ambulance station in Soundwell, Bristol at 7am on Sunday.
Her vehicle wasn't the only one targeted, with one of her colleagues finding the same on their car.
Although the graffiti has been removed for free by a local company, NHS worker Bronwen wanted to speak out about how the incident affected her.
Bronwen said: "It says 'don't call am 999'. I think they were going to write 'ambulance' and changed it to '999'.
"The girl next to me also had her car painted.
"It was in the staff car park, so they would have had a really good idea that we were working for the ambulance service.
"She had two or three big pink dots down one side.
"It made me feel pretty angry more than anything and disappointed.
"I'd just done 12 hours in work for my fourth shift in a row in like six shifts in six days.
"You feel like you're making a difference and helping people and that's what spurs you on through the long hours.
"Then you get back and that's how your help and your hard work pays off. I didn't know whether I was upset about it or angry about it.
"I was trying to put a brave face on it because quite a few people were around. Initially I was quite upset and then when that subsided I was really angry about it.
"When I got in the car to drive home, I got a bit emotional.
"I phoned my partner to let him know what had happened and just got a bit teary."
On Saturday night, some of our colleagues working a night shift at Soundwell Ambulance Station, Bristol had their cars vandalised in our car park. This is #unacceptable, @ASPolice have been informed. If you know any additional information please call 101 & quote CRN 5221060379.
- South Western Ambulance Service (@swasFT) March 22, 2021
Bronwen, from Caerphilly, South Wales, shared the vandalism on social media, receiving support from grateful friends, family and members of the public who appreciate her hard work.
Since the incident, her car was cleaned by Matt Churchward from Spray Coating Specialists, who saw the post and wanted to sort it out for her.
Bronwen's employers, South Western Ambulance Service said on Twitter: "On Saturday night, some of our colleagues working a night shift at Soundwell Ambulance Station, Bristol had their cars vandalised in our car park. This is unacceptable, Avon and Somerset Police have been informed.
"If you know any additional information please call 101 & quote CRN 5221060379."
Avon and Somerset Constabulary confirmed the damage has been reported and are appealing for anyone with information to get in touch.