The What a Girl Wants actress apparently arrived at clothier Kin just after 3.30pm local time and stayed in a stall for close to two hours, reports TMZ
Bynes is thought to have told employees that she needed "more time" each time they checked on her. She ultimately emerged, paid for some items and then left in a taxi, according to a witness.
The website quotes a store clerk as saying: "I was so worried about [Amanda]. I didn't know what she was doing in [the dressing room]. I didn't know if she was overdosing, or doing drugs or what."
Bynes's personal life has been the subject of many media reports over the last few months.
Her car was impounded over the weekend after an arrest on suspicion of driving under the influence and several car accidents.
Bynes was rumoured to have checked into rehab on Monday, but that claim has since been denied.
She is expected in court in LA on Friday to answer one of her drink driving charges.