There are many different forms of amnesia. It's not always total wipe of the memory. There is a type where you lose a certain timeframe. There is selective amnesia. There is also one called localized amnesia where you lose specific timeframe or event(s) in your life.
There is this show on Fox called Doc, and the lead suffered a brain injury and as a result, she cannot remember the last 8 years.
Anywho, that being said--it likely isn't amnesia but they need to come up with a good reason other than what she told Jason, which was she didn't want to burden anyone as HD progressed--she wanted to die alone. Guess what? We haven't seen any tremors at all.
They have a lot of questions to answer:
Why would she allow Liesl of all people to think she's dead?
How did she manage to not have a pulse/stop breathing when she died in Liesl's arms? It's not like she new she would encounter The Hook that night.
