The Hairspray star has sparked concerns for her wellbeing in recent months after posting several topless shots online and being arrested in New York last week (23May13).
She has also been stirring up trouble on Twitter in recent days, taking aim at blogger Perez Hilton over an article he posted and branding Chrissy Teigen "an old ugly model" after the beauty questioned the troubled star's mental health.
Now she has turned on Bass, who produced her 2005 movie Love Wrecked, after he spoke about her apparent issues.
In an interview on a Las Vegas radio station, he revealed, "No one is able to get to her. I'm actually really scared for her, and I hope someone is able to get to her to give her the help she needs."
Bynes heard about the former 'N Sync singer's comments and took to her page on the microblogging page to declare, "I ignored Lance Bass on Twitter and now he is saying I have a mental illness. Sorry you're an ugly ex boy band member w/ (with) no talent or career."
Bass fired off a response, stating, "Ouch - The Amanda I know would never had said something like this to me - either this is a fake person or she really does have a mental illness. Way too sad. I reached out - all I can do."
Bynes later added, "I haven't spoken to Lance Bass in 11 years. To say I have a mental illness out of nowhere makes me lash out! You're making me crazy!" prompting the singer to declare: "Now I really don't think this is Amanda - we shot LoveWrecked together 7 years ago and she was just at my bday (sic) party 3 years ago. Hmmm..."
Bynes' rant comes a week after she was taken into custody at her New York City apartment and later charged on suspicion of marijuana possession, attempted tampering with physical evidence and reckless endangerment, after allegedly throwing a bong out of the window.