He got a little prod from the TARDIS that someone was at the door, right in the middle of that kiss, and his first thought was that the TARDIS was just jealous and trying to get him away from Jack.
Then he remembered Romana and that he hadn't gone to find her yet, and he pushed Jack off with gentle urgency. "Romana's at the door," he explained. "I'm just going to get her. Why don't you get some more rest?"
"There's a Master in my bed," Jack quipped, getting off him nonetheless. He looked a lot better than he had done, but he was very pale, still, and shaky.
"Take mine," the Doctor suggested, because Jack was going to rest even if he had to sedate him again to make him. He thought for a moment that Jack was going to argue; watched Jack assess his condition and think better of it.
He nodded reluctantly. "Okay. Go and talk to Romana. Don't leave without telling me what's goin on."
"I promise," the Doctor said, and set off for the console room. He opened the door to Romana and immediately started apologising. "I'm so sorry, Romana - I completely forgot about coming to find you! Some... other stuff came up this morning."