Lewis Hamilton says he has been "fighting all year" with his health, after contracting Covid-19 in December.
The seven-time world champion saw his Mercedes team's doctor with dizziness and blurred vision after finishing third in Sunday's Hungarian Grand Prix.
After belatedly joining the post-race news conference, Hamilton said: "I had really big dizziness and everything got a bit blurry on the podium."
Hamilton missed last year's Sakhir Grand Prix because of coronavirus.
He returned for the final race of the season in Abu Dhabi a week later, but admitted he was not fully healthy when he did so.
After finishing Sunday's eventful race at the Hungaroring, won by Alpine's Esteban Ocon, Hamilton took part in the podium ceremony before seeking medical attention.
"I have been fighting all year with my health after what happened at the end of last year and it is still a battle," he said.
- Ocon wins thrilling Hungarian GP after Hamilton fights back
- Full race results
- Vettel disqualified from Hungarian GP
Hamilton said he suspected he was suffering from long Covid, which causes people who contract the virus to suffer debilitating health effects for months afterwards.
"I haven't spoken to anyone particularly about it but I think it is lingering," he said. "I remember the effects when I had it. The training has been different since then and the levels of fatigue you get are different and it's a real challenge.
"So [I'm] just continuing to try and train and prepare the best way I can.
"Who knows what it is today? Maybe it's hydration but I've definitely not had that experience. I had something similar in Silverstone but this is way worse."
Hamilton's third place coupled with a 10th-place finish for title rival Max Verstappen of Red Bull after the Dutchman was involved in an accident at the first corner of the race has left the Briton with a six-point championship lead heading into F1's three-week summer break.
However, the gap will extend to eight points if Sebastian Vettel's disqualification for fuel irregularities is upheld following appeal.
The season reconvenes at the Belgian Grand Prix on 27-29 August.
A further 12 races are planned but there remains uncertainty about the exact number as a result of the pandemic.
- The man who introduced Joe Wicks to meditation: Exploring spirituality and mindfulness with Russell Brand
- The Ancient Olympics: You're Dead To Me explores how even death couldn't stop you winning