Christensen was a surprise omission from the Chelsea line-up to face Brighton at Stamford Bridge having started the last nine league games.
The Denmark defender was replaced by Gary Cahill, who made a rare start in the centre of defence.
Without Christensen, Chelsea were still able to comfortably beat Brighton thanks to goals from Alvaro Morata and Marcos Alonso.
But after the match, Conte confirmed Christensen was suffering from illness.
Conte also expressed his delight with his players for breaking down a tough Brighton defence.
He said: “In the first half we had the right chances to score but didn’t take them. For this reason it becomes difficult because you’re opponent stays with 10 players behind the ball.
“In the second half we settled very well and created many chances. It was a good performance with good intensity.
“It’s the same performance against Everton – against Everton we showed 25 times we had shots on goal but today we scored twice.
“We are in a good moment of form and just continue this way.