The troubled singer - who has a history of drug addiction and heavy drinking - checked herself London`s The Priory clinic earlier this week - but reportedly stunned onlookers when she bought a small bottle of vodka from a small shop in Southgate, north London, on her way to the facility.
The source said: "She seemed out of it. She was stumbling about, slurring her words.
"I was shocked to see her buy vodka so early in the day, and even more shocked to see her knock it straight back."
Just minutes earlier, she had shocked staff and clients in a local hair salon by bursting in and loudly vomiting in their bathroom.
Customers said she was dressed scruffily in a tracksuit and told staff: "I just puked all over your bathroom."
One told The Sun newspaper: "I felt really sorry for the staff. I think they were too scared to say anything."
On 26.05.11 Amy`s spokesman denied she had been sick, insisting her comment was a joke, but confirmed she is in rehab.
He said: "Amy`s embarked on treatment at The Priory."
Earlier this month, it was reported that the 27-year-old star wants to move to the countryside and start a family with her boyfriend Reg Traviss.