At the beginning of the episode, we learn that a sober Spencer has just returned from a 3 day stint at rehab. This is a big change from "She's Come Undone" when she was taking adderall "prescribed" to her by "A." Taking the medication that "A" gave her was not only an enormous lapse in judgment, considering it could have been poisoned, but also illegal! Spencer's poor judgment brings us to:
Broken Law # 1 Alert! (from episode 4x21)
Controlled Substance, Drug, Device and Cosmetic Act (35 P. S. § § 780-101—780-144) (in relevant part)
"...Knowingly or intentionally possessing a controlled or counterfeit substance by a person not registered under this act, or a practitioner not registered or licensed by the appropriate State board..."
Spencer has more of her terrifying flashbacks to the night that Alison disappeared. These flashbacks strongly indicate that Spencer hit Alison, or somebody who looks a lot like her, with a shovel...hard.