From Fanlore on the topic:
The Hanahaki Disease trope was popularized with the Japanese shoujo manga,「花吐き乙女」(Hanahaki Otome), or The Girl Who Spit Flowers by Naoko Matsuda (松田奈緒子), which was released in 2009. The symptoms of the disease are summarized to strong pain, having flowers blooming in the heart and lungs, and then throwing them up.
However, among East Asian (Japanese and Korean especially) fans and creators, the concept of flower regurgitation due to unrequited love dates to before Hanahaki Otome's release. Its true origins are currently unknown.
Can anyone find an instance earlier than 2009, and maybe find the earliest?
Why am I asking? Because TVTropes is talking about it.
Expect your answers to go there...
And, KnowYourMeme isn't giving me anything about Hanahaki Disease.
Urban Dictionary's entry is like, from 2018? Way too late.