Moroboshist

The flu seems to have hit quite a few of the members recently. It's likely that whatever germs they have encountered might have passed to her. They have been interacting with each other a lot because of their rehearsals and their performances. That'd be my only guess, but like I said, it could pretty much be anything that set it off.
But it's probably more likely that she won't be back until full recovery because it's in her neck. It won't be easy or comfortable to be singing, dancing, jumping, and running around for hours and will probably just make it worse, even if it has shrunk down a bit. We don't know for sure though, so we just gotta wait 'til the kid's back on her feet.

Just gonna toss in my share of info here.
I've had a swollen lymphnode before when I was 12 right under my chin.
You have lymphnodes all over your body and swelling occurs at those sites where an infection or bacteria could potentially be.
Mine was particularly painful and was extremely uncomfortable the first few days, but after I went to the hospital and was prescribed my medication, it wasn't so bad.
I was lucky that I didn't get too high of a fever.

At most, Masaki will experience discomfort, and depending on where the lymphnode is on her neck, there could be some pain if she puts too much pressure on it. But it's not as bad as the name suggests. As long as she takes her antibiotics regularly (and continues to take them until they are gone), her fever is kept under control, and maybe if her parents got her some painkillers, she'll be perfectly fine. Swollen lymphnodes are actually pretty common and it's just the body's way of fighting off the bacteria or infection. That infection or bacteria could come from virtually anything. The most she'll be affected by this in terms of her work is missing a big portion of the current concerts. Nothing more, nothing less. Poor girl will be fine.

Her hair really does give her an edge and makes her look older.
I liked her hair back when it was shoulder length too, but she looks really good in shortcuts.
She pulls them off well.

^She is beyond precious in that picture!

She has been looking very down recently, or at least her attitude seems off.
I'd like to think she is just trying to be focused and little more serious since the older members are leaving and there will probably soon be more focus on her as an established senpai already, but that's probably me overthinking it. LOL.