So, Megan was clearly infected, but what stage were they implying she was? She wasn't stage 4, because when she woke up she should have attacked her mother.
If she was stage 1, why did her heart stop beating the end? Did they shoot her?
My impression is that Megan was recently infected, so in stage 1, which it's implied can be cured.
I think she is drifting in and out of consciousness because of dehydration, because she ran out of water days before her mom rescued her.
The stopped heartbeat effect was, I think, her mother, who had been shot sever times, dying. You hear a voice on the radio asking for the ID number from the suit, and when its given, saying that she's Dr Chase, the last surviving CDC doctor, and to rush her to treatment.
Her mom's scheme to save her her, even at the cost of her own life, succeeded.