"What Miley is suffering from is much more than an allergic reaction to antibiotics," a source said.
"She’s really suffering from the death of Floyd. Floyd was her best friend. She feels like a part of her, a big part of her, has died."
Heartbroken Miley took to Twitter at the beginning of April to announce the sad news that her beloved dog Floyd had died.
"I don't wanna say it because I don't want it to be real… But my precious baby Floyd has passed away," she wrote.
"I am broken."
Miley introduced fans to the adorable Alaskan Klee Kai in 2011, writing on Twitter, "I'm so in love with him it is ridiculous."
Miley regularly posted pics of Floyd on social media
The 21-year-old is currently touring the US on her Bangerz tour, and will soon be bringing it over to the UK.
She was hospitalised on Tuesday, and tweeted to her millions of Twitter fans: "Kansas I promise I'm as heartbroken as you are."
She added: "I wanted so badly 2 be there 2night. Not being with y'all makes me feel s****ier that I already do.”
"Mr Octopussy & some amazing Drs are taking good care of me," she added, next to a selfie from her bed.
Miley in hospital this week
Speaking to HollywoodLife, the source said: “She hasn’t been sleeping and it’s caught up with her. She is exhausted, beyond exhausted physically, mentally and emotionally, and her family is concerned.
"She really hasn’t had any time off and she would have collapsed on stage had she kept going — she needs liquids, hydration, and most importantly, sleep. The sleep deprivation has caused massive havoc on her emotional well-being."