Free Speech Coalition executive director Diane Duke told The Los Angeles Times on Monday that her group became aware of the HIV case Saturday.
A series of tests were being conducted on the performer to confirm the case before anyone the performer might have spread the illness to will be notified to get tested, Duke told The Associated Press.
She didn't know how long that would take.
Duke declined to release the performer's name, age or gender, citing the person's federal right to medical privacy. She also declined to say how her group learned of the case.
The case was found in an out-of-state clinic that doesn't report to California health officials, said Duke.
If the initial case is confirmed, the group will ask two generations of the person's sexual partners to get tested, meaning those who had sex with the performer and the sexual partners of those who had sex with the performer.
The voluntary industry shutdown affects porn producers in the San Fernando Valley, the heart of the multi-billion dollar American porn industry, and includes Hustler and Evil Angel's productions. The porn industry was shut down similarly in late 2010, after porn actor Derrick Burts was diagnosed HIV positive.
Burts has since gone on to advocate for the mandatory use of condoms in porn with the AIDS Healthcare Foundation.
The health advocacy group and state workplace safety officials say state law mandates porn performers to use condoms to protect themselves under the same set of rules that require nurses to wear gloves in hospitals when dealing with bodily fluids.
Cal/OSHA is working to clarify the regulation to make it more specific to porn.
Earlier this month, the health advocacy group announced that it will gather 41,138 petition signatures to get the issue of condoms in porn on the June 2012 ballot.
The ballot measure would ask Los Angeles residents whether porn producers must require performers to use condoms on shoots as a condition of getting a filming permit.
"The question remains how many performers must become infected with HIV and other serous STDs before the industry will clean up its act and government will do the right thing?" said Michael Weinstein, president of AIDS Healthcare Foundation.
The group has unsuccessfully pushed California and Los Angeles County officials to tighten enforcement of condom use on porn sets through legislative attempts, lawsuits and regulatory complaints.