Gura is in rehab for prescription drug abuse. It's all starting to make sense.
Gura has had physical health issues for a while including sleep disorders. She started seeing a doctor and was prescribed prescription drugs. But somewhere along the line, she developed an addiction to them and has been abusing them since. Her streams have gradually become fewer and further in between due to her descent into pill-popping. When she does stream, her heart rate is redlined because she's sky high on prescription pills well beyond her assigned dose. The reason her collabs tanked after that Myth off collab was because the other girls noticed her heavy usage of pills and tried to tell her that she had a problem. She got angry at them and insisted she was fine, but it caused some awkwardness and apprehension between them and they felt less comfortable collabing. Finally it's gotten to a point where her family got too concerned and scared for her well being and are forcing her into treatment. She finally came clean to management and they are putting her on hiatus until she's out of rehab.
Gura is under a great deal of stress from her online presence, having to put on a show for the viewers and put on a brave face even when she's feeling down. She began abusing prescription drugs as a way to cope with this pressure, leading to addiction, dangerous side effects, and now her latest hiatus.
TLDR: A lot of anons have speculated that Gura is pregnant because she hasn't discussed the nature of her issues in much detail. The truth is that she is in drug treatment and can't discuss it openly because it will tank the reputation of Hololive.