Miley Cyrus -- Death of Dog Triggered Illness Triggered Hospitalization

Miley Cyrus may have been rushed to the hospital because of a bad reaction to antibiotics, but we're told the reason she was on the meds was because she wore herself down by hard partying and an emotional meltdown over the death of her dog.

Sources connected to Miley tell TMZ ... she has been "an emotional mess" since her dog Floyd died earlier this month. We're told she had crying fits and has been unable to sleep.

Our sources say Miley's escape has been partying ... as one person said, "She's been off the rails since this happened." We're told she's been drinking a lot and drugs are also involved.

We're told Miley was on meds because she got sick from being extremely worn down.

Everything's connected to everything.

We reached out to Miley's rep ... so far no word back.