Ricardo Gomez confirmed to Radar Online that 21-year-old Selena was diagnosed with the auto-immune disease "a couple of years ago" but is now coping well with her symptoms.
"She's in treatment," said Ricardo. "She's getting along pretty good."
Ricardo added that despite his granddaughter's illness what worried him more were media reports suggesting Selena was taking drugs and the fact that she rarely takes time off work.
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"We just saw in some newspaper that she was doing drugs. I called her and said, [you] need to watch out," he said. "She was working a lot. She was exhausted. She hardly takes some time out.
"There was some pressure, first because it's a lot of work," he added. "The doctor was telling her she needs to take some time out. It's just too much pressure what she's going through. It's just too much! She doesn't have a single day's break... It's getting to the point with all this pressure... that it's too much.
"Everyone was telling her, even her agent and the doctor, 'Selena take some time to yourself!'"
Selena is currenty in Nepal working for UNICEF
Selena, who is currently in Nepal working for UNICEF, was raised by her grandparents Ricardo and Mary for about three or four years while her parents were still in high school.
"It was kind of a hard start for them," said Ricardo. "They had dated for I would say two years prior to that. They went wild and [the pregnancy] happened.
"We kind of raised her," said Ricardo. "Selena's father, he was always working. Her mom, Mandy, too was real hard-working when she was a young lady."
Despite growing up close to his granddaughter, Ricardo said that Selena's hectic schedule means he rarely sees the Springbreakers actress.
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"The sad part is we hardly see her nowadays," he explained. "Just by phone …When we talk on the phone we say, 'We don't see you in person but we see you in the news, in the newspaper, the TV… Are you ok?' She says, 'Yeah.'"