Shaw is outright implanted with a chestburster, not a hybrid (which I thought was a better, more twisted idea). She's in the medi-pod just as its going off, so she sort of half-chestbursts, half-gets it removed....basically its surgically removed but its so mature that it does a LOT of damage on the way out. But the nifty thing is that its like getting chestburstered right in the middle of an ER: one of her lungs has collapsed and she has massive internal bleeding, but the auto-doc INSTANTLY sets about trying to repair her, and she passes out for several hours while heavy surgery fixes her up.
However, the chestburster escapes through a vent and into the main room. In the meantime, all hell is breaking loose on the ship as the Holloway-Chestburster (which is a lot more gooey and can slide *past* grates into vents, must be a different species) is doing a lot of damage. Hours later, one of the hidden soldiers (not really bad guys) rushes in and seeing Shaw there tries to check on her, but gets ambushed from behind by the chestburster; not metamorphosizing into a young Xenomorph warrior.
So it isn't being removed while another alien is in the room; he escapes, a few hours pass while she's in surgery, and its in the room outside.
The "twist" is that as Shaw regains consciousness, she sees the autodoc arms still working on repairing her abdomen, and sees the Alien warrior outside chewing up the soldier's body....and that he dropped his gun right outside the medipod. Shaw realizes that when the medipod is finished with surgery, it will open (for the moment the Alien is ignoring all of the motion inside of it). So even reading it, its this REALLY nailbiting scene, race against time, intercutting between if the Alien notices her, the timer on the surgery winding down, and then ultimately the pod opening. At the last moment, as the pod opens Shaw is able to grab the soldier's gun and empties a clip into the alien, killing it. She then fiddles with the dead soldier's pack to find an extra clip, reloads, and sets off for the bridge. So imagine how kind of worn-out but tough Shaw looked trudging through the hallways post-surgery, bloody and in bandages, but having just blown away an alien and reloading a clip into a gun.
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