While viewers didn't find out what was wrong with the 50-year-old reality star (who has previously been in rehab for alcoholism) she insisted on her Bravo blog that she was suffering from bronchitis and pneumonia before the infamous poker party thrown by Eileen Davidson and her husband Vince Van Patten.
During the party scenes that aired on last week's episode, Kim admitted to sister Kyle Richards that she had taken "something" for pain, which bothered Kyle so much that the party ended with a fight between the sisters.
And as Tuesday's episode picked up, their co-star Lisa Rinna -- who had previously asked Kim if she was drinking, which Kim denied -- sighed, "When sisters collide, I don't know what the f*ck to do."
During Rinna and Kyle's ride home from the party, Kyle, 46, revealed her concern about how Kim was dealing with the illness of her ex-husband, Monty Brinson.
Rinna warned Kyle, "I know your sister is a sober person, but she did not seem sober to me."
"I know what Kyle is going through because I saw Harry watch both of his brothers drop dead of alcoholism within six months of each other," Rinna said. "Unless anyone wants help, you can't help them."
Later, a frantic Kyle revealed to husband Mauricio Umansky, "Kim is in the hospital," but she didn't know why, only that, "She was in a lot of pain and Monty took her to Cedars-Sinai."
Although she hadn't been able to speak with Kim, Kyle knew that her doctors "said that she could have an ulcer.
"But I don't know if they saw an ulcer or if they're guessing from her pain or her reaction."
Kyle told the cameras that she's seen her sister "go through a lot of difficult times and be under a lot of pressure.
"I'm worried, I'm frustrated. I don't know what is going on with my sister. She was not herself and I know she was in a lot of pain, she said, and she took something." (The show flashed back to last week's episode, in which Kim told Kyle she was in pain and before she left the house, "Monty gave me something for the pain that my doctor gave me and I took it.")
While Kyle admitted sibling relationships are often complicated, she added, "I think with Kim and me, there's a whole other dynamic because of her sobriety and my not knowing how to handle it always."
Kyle told her husband she thought helping cancer-stricken Brinson was "too much" for Kim.
In a later scene, pal Yolanda Foster visited Kyle and commented, "You have so many dogs. And they're all so friendly."
But on Tuesday's show, after Foster pet Kyle's tame dogs, she learned what was going on with Kim.
"She was doubled over in pain and ended up in the emergency room," Kyle said.
Yolanda had sympathy for Kim, noting, "With her daughter getting married, her other daughter moving out, nursing Monty every day through what he's going through, it is so hard. It's a lot of pressure for anybody."
At the end of the episode, Rinna threw a jewelry sale party and Kyle was again peppered with questions about Kim's hospitalization.
"Right now they are doing all these tests," she said. "I went to the hospital and waited an hour and a half today and she never came back to the room.
"Her kids and I were all waiting and they were sending her down for more and more and more" tests.
Kyle said to the cameras, "I feel like I'm in a tough situation and everybody wants to know what happened with your sister. Is everything okay? I don't have that answer right now."
On Tuesday's episode, fans also saw Foster talking on the phone to her daughter Bella, who is now attending college in New York.
Bella, who was arrested for DUI last year, showed her Hollywood pedigree when she informed her mother she was going to an NYC art show with "Tallulah and Demi," as in Demi Moore and her daughter Tallulah Willis.
Foster thought it sounded like fun, but the protective mom, who was bitterly disappointed about her daughter getting into DUI trouble, reminded Bella of her curfew.