Interesting set-up. Reminds me of Invasion of the Body Snatchers. I felt like you'd jumped too far, too fast, moving from the "Gosh, is something up with Ashley?" phase, through a neck-breaking segue to all hell breaking loose at the end. I think there's a step missing there, somewhere.

Possibly the problem is in the way that much of the preparation material was merely talked about, in a conversation between Hannah and Rachel. Rachel, actually, has not a lot to do with much of anything other than to give Hannah a reason to tell the audience things that you think they need to know. Never a good use of a character. Were you to reassign Rachel to the background, where she belongs, and rework the preparation material into actual interaction involving Hannah and Ashley, you'd bring your reader along with you much more effortlessly.

Another problem I have is that Hannah actually has no story of her own, other than to be on the receiving end of events totally beyond her control. There's nothing in particular to involve us with her arc. It wouldn't take much. Just something that constitutes a goal for her, something that the unfolding events complicate. Something to make us wonder, "What's she going to do now?"

I won't comment much on the final scene other than to note that it seems to pack the desired punch, disgusting though it may be. Lacking the necessary prep work, as I've already pointed out, the final gore seems to come out of no where. I suspect you could have served the requirements of your plot just the same without sending half your audience running from the room, trying to stifle their gag reflex.

Still, it's a decent idea at its core. Certainly worth refining.