Octomom -- The Tax Man Cumeth ... For Porn Pay

The IRS has learned a thing or 2 about self-pleasure from Octomom ... because the agency is now pleasuring itself by going after her unpaid taxes on her masturbation video.

Uncle Sam and his buddies believe Nadya Suleman raked in at least $116,000 in 2012, largely from the tape. They're demanding that she ante up all payroll records to show exactly what she made ... and they clearly think she shorted them.

Octo caught the eye of the feds after she pled no contest to a welfare fraud charge in July.

It looks like the wacky tape didn't have legs, because in 2013 her take plunged to around $8K.

Tax records show Suleman collected about $116K from her management company in 2012.