The tweet in question, posted to the official ONEFORJASMINA account this weekend, read: "It has been a month since Jasmina's passing. Still not even a card or text message from Rihanna. Did she use my child for a publicity stunt?" The note has since been deleted, but was reposted a number of times by other Twitter users.
"Apparently there are some negative blogs out there. For the record, Rihanna has been nothing but really nice!!!" read an updated tweet Monday afternoon.
In an e-mail to PEOPLE, Thea Anema denied writing the message herself, but said she is disappointed not to have heard from Rihanna, 22, since Jasmina passed away on Jan. 27.
"Friends have helped me with all kinds of stuff and many know the passwords to my personal e-mail, the Caringbridge site and also Twitter," Anema wrote. "Although I did not Tweet this particular one, I do wish that Rihanna had responded to me on Jasmina's passing. It would have meant the world to me."
Still, Anema says she is grateful for everything Rihanna did for Jasmina.
"Jasmina adored her," she writes. "Rihanna was so kind to come and visit her. I just felt hurt not hearing anything from her. Or of any other celebrities for that matter. The only ones who reached out to me personally during this time of grief were the Obamas."
Friends of the singer have come to her defense. One source close to Rihanna tells PEOPLE: "She really loved this little girl. It was hard for her to deal with a child passing away, especially a little girl she had come to care for so much."
Days after Jasmina's death, Rihanna remembered her fondly, telling the AP, "She was a very free, fun spirited little girl, full of energy, always happy, always energetic." "I always remember she wouldn't sit still. She was always running around, always had a smile on her face."
In April, Rihanna is set to appear in PSAs for DKMS Americas, an organization she's worked for since 2007. It is the world's largest marrow donor center, which helps leukemia patients. In a statement, DKMS co-founder Katharina Harf said Rihanna's work for the cause is hardly a publicity stunt.
"Last year she recruited over 5,000 bone marrow donors to DKMS – resulting in lives saved," Harf said. "She is relentless in her campaign to help children in the fight against leukemia and continues to urge her fans and people around the world to register with DKMS."