My twin sister has a throbbing headache on her right side, what could it be?

Hi! This sounds like a condition I suffer from called Hemiplegic migraine. It’s a type of migraine with symptoms that present very similar to a stroke. The severity of the symptoms vary from person to person, mine are generally very mild but I know people who lose vision and other senses completely when they are having an episode. She should definitely consider seeing a neurologist or headache specialist. If she takes any Triptans for migraine she should stop using those and call her doctor immediately, as triptans can trigger a stroke or other threatening health conditions for individuals with hemiplegic migraines. So sorry you and she are walking through this, I know it’s a terrifying feeling! Best of luck to you both as you seek a diagnosis.

I'm not sure what this is (sorry), but I do agree it sounds very scary & def recommend she see a doctor about it. I would be worried about a stroke by the sound of it