Michel can be seen grimacing with pain in a picture taken as Astrid, the younger sister of Belgium's King Philippe, fired the blank next to his left ear at a 20-kilometre (12-mile) race in Brussels on Sunday.
The French-speaking prime minister had to postpone an appearance in parliament to present a national investment plan todayto undergo tests, his office said.
Michel was suffering from "partial deafness" and "tinnitus", or ringing in the ears, his spokesman Frederic Cauderlier told AFP, declining to give further details to "maintain his privacy".
Michel underwent extensive medical tests on Monday and Tuesday morning in a bid to treat the problem "as soon as possible after the incident," Cauderlier said.
The incident comes a week after Michel hosted US President Donald Trump, who met him and the king before attending summits with the EU and NATO.
Trump used the visit to give NATO allies an ear-bashing for not paying their fair share for defence costs.