"James O'Connor felt unwell in the bus taking the team back from the Oyonnax game (last Sunday)," Toulon coach Bernard Laporte told RMC radio.
"The medical staff decided to take him to hospital and he underwent a considerable series of tests."
French media reported that O'Connor had suffered a heart attack.
"I can reassure everyone he's fine," said Laporte. "He will play rugby again but he will undergo further tests next week.
"James will miss three weeks, maybe four weeks of competition."