Lucy Till, aged 24, even told the guard he had caused her to have a miscarriage – which the judge said was 'plainly' not true.
She admitted to shoplifting at Decathlon Super Store, Wednesbury, when questioned by police on February 10.
Mr Howard Searle, prosecuting, said the following day Till logged in to her Facebook account, found the personal page of the security guard who was a witness in her case, and sent him a message through the social network saying: "You dirty f****** little grassing b****** ruining people's lives.
"I suffered a miscarriage this morning due to stress. I don't know why you couldn't just give me a ban you've got your s*** back what's the problem? You cost me a life so I will ruin yours."
Till, of Millfield Avenue, Bloxwich, was interviewed by police on March 10 for the threats and for another shoplifting offence, and said she had stolen items as she was pregnant, but declined to answer questions about the intimidation.
The single mother pleaded guilty to intimidating a witness at an appearance in Wolverhampton Crown Court.
She had claimed the arrest caused a miscarriage but the date was disputed, the court heard.
She has previous convictions for shoplifting and handling stolen goods, the court heard.
Mr Stephen Cadwalader, mitigating, said she was in a relationship that failed, adding: "She couldn't get childcare and gave up work and ended up as a single mother, and with little support she came to suffer depression."
Till, who has a five-year-old daughter, remains on medication for depression and anxiety.
She was recently in a violent relationship with another man, and told Mr Cadwalader she miscarried when she was released from the police station. He said: "As far as the motivation for sending the message, it wasn't to get away with the criminal offence – she had already admitted it to police – it was a moment of utter stupidity at a time of high emotional stress."
Coming to her senses, she attempted to send an apology but was 'unsurprisingly blocked'.
Recorder Mr Barry Berlin said: "Anybody who intimidates a witness must go to prison. If we don't protect witnesses, courts just grind to a halt."
"It would be bad enough calling him what you did, but you say because you had a miscarriage that morning, which you plainly didn't have, that he was responsible for it."
Till was sentenced to nine month's prison, suspended for two years, with an 18 month rehabilitation activity requirement, and 100 hours' unpaid work.