"I thought, `It's a nodule, I'm a guy, I don't think it's anything more than that,'" he said. "The more I messed with it, the bigger it got and the more it hurt, and that started really scaring me."
The former Kiss drummer went to the doctor, underwent some tests and a surgical procedure to remove the lump. A week later, the doctor called. It was breast cancer.
"My heart hit my stomach and my knees buckled," Criss recalled.
The good news was that Criss had caught the disease at its earliest stage. After a second surgery to remove it in March 2008, he would not need chemotherapy, radiation or medication.
Now, the once-costumed rocker who performed in his Catman makeup is speaking out about his illness to encourage other men to get tested for breast cancer - a disease more commonly associated with women, who are routinely urged to get regular mammograms at a certain age - the moment they suspect something might be amiss.
"You need to immediately tell your wife, your girlfriend, your boyfriend, whatever," he said. "The more you sit around and say, `Well, it's going to go away,' that time could be the time that you save your life."
Men account for only 1 percent of all breast cancer cases, but about 2,000 men develop it each year, and 440 die from it, according to the National Cancer Institute.
Dr. Alexander Swistel, who treated Criss, praised his decision to get checked out immediately after sensing something was wrong.
"He's a great spokesperson, and he's very bright about this kind of thing," Swistel said. "To have someone like him come forward and show that there's life after this is a wonderful thing. Rather than be the typical guy and say, `Ah, forget it,' he moved on it right away."
There should be no stigma attached to having the disease, or seeking help for it, Criss said. Other tough guys, including "Shaft" actor Richard Roundtree, have spoken out about having breast cancer.
"It's something we think guys don't get," Criss said. "Guys get prostate cancer, heart attacks. Men are men - women get breast cancer. Or so I thought."
"It has nothing to do with macho," said Criss, best known for the 1976 ballad "Beth," which remains Kiss' biggest hit to date. "There's no tougher guy than me. I was born in Brooklyn, I was in gangs, I did the whole battling thing my whole life. I think a man is a man if he steps forward and says, `There's something bad going on and I need to deal with it."
His doctor says the 63-year-old Wall Township, New Jersey, resident is cancer-free today. Criss' treatment gave him an up-close look at what women have endured for ages.
"I sat in the waiting room, and there were all these women who looked like they weren't going to be here long - no hair, scarves - a place a guy doesn't think he's ever going to be sitting in one day," he said.
And having a mammogram was an experience in itself for Criss.
"It's amazing how they can get a guy's little pecs in that thing that the poor women go through," he said. "They are so medieval! I have a whole new respect for women going through mammograms."
Criss was a co-founder of Kiss from 1973 to 1980, did a reunion tour from 1996 to 2000, and returned for a final stint in 2003. He's working on a new solo album and a long-delayed autobiography.
"I am the Catman, and I do have nine lives, but I think I'm down to five now," he said.