The two most prominent theories right now are that
1) As he is having a growing impeachment crisis right now, he had chest pains that were either a mild heart attack or a panic attack, and
2) He figured out that the 25th amendment was actually written to remove a president who is physically incapacitated - it was ratified after John Kennedy was assassinated - and not just mentally incapable, so he is coming up with a physical ailment that might justify his retirement and Mike Pence issuing a blanket pardon for the entire Trump crime family.
If it turns out that theory number 2 is accurate, it wouldn't be the first time that Donald Trump has used a doctor's report to get him out of something; there's a reason Senator Tammy Duckworth calls him Cadet Bone Spurs.
Plus, In other news, Are We Going to Let Capitalism Kill More of Our Children?
Smoking kills and the worst part of capitalism is that our system allows a half million Americans a year to die from their nicotine addictions. Now Donald Trump has decided, after a seven week multi-million dollar lobbying campaign by tobacco and vaping companies, that is just fine for these companies to sell candy flavored Vape to children. Trump has decided that unregulated capitalism is more important than a few million dead teenagers over the next few decades.