In the initial sparks of Shellie's awakening, she inadvertently altered her nervous system to accept an expanded set of information: light and sound outside the normal spectrum, smell, touch and taste with more granularity, and all with incredible sensitivity. Unfortunately, she quickly wound up burning out and turning the power off, leaving her practically blind and deaf as her mind is unable to process the information correctly. Visible light is compressed into shades of green, sound is magnified and distorted beyond recognition, chocolate tastes kind of like cocoa bean mulch.
Over time, she has progressed to the point where she can make out the world around her, and has moved on to deciphering the infrared yellows and ultraviolet blues that constantly haunt her vision. All this, and all analysis still shows her as mundane, albeit with a mutant sensory nervous system; she's not been living in the sort of stressful environment that would trigger another awakening event, though no doctor is exactly expecting one in any case.
Full power description:
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Level 0: Her current level. Light and sound are compressed in their frequency spectra, new sensations are distracting, and loud noises are painful. Her powers are as often a liability as an asset.
Level 1: She finally gets a complete grip on the spectrum of information that her senses are giving her. She can distinguish light and sound frequencies as well as she could before, and can do the same with the new frequencies and smaller amplitudes she now has access to. She can distinguish between individuals by smell and can sometimes tell where a given person has been if they're particularly stinky. She starts to develop new nerve endings throughout her body, creating a more granular sense of touch that's mostly good for catching ticks and leeches, or maybe hearing conversations between whales and elephants.
Level 2: She learns to filter through sensory information for particular sights and sounds. With concentration, she can make out fine details as though using binoculars, listen to a whispered conversation on the other side of a crowd, or sniff out your lost cell phone in a pile of garbage.
Level 3: Her increasing sensory processing power readily does all of the level 2 feats in an instant. She can see your arm begin to tense up before you pull the trigger and twist aside, creating the illusion that she can dodge bullets. She can count the change in her bag without reaching her hand inside.
Level 4: With concentration, she can see great distances as though with a high-powered satellite or sniper scope. Close up, she make out the individual skin cells on the back of your hand. Her nose can match such skin cells up with the person they came from. If she can pick out your voice in a crowd, she can track it as you walk across town. She can feel her own hair and skin cells routinely sloughing off if she chooses to pay attention. She can count the change in your pocket without so much as looking at you. She can track burrowing creatures by vibrations in the ground. She can listen to your heartbeat from anywhere in the building. She can look at you, through a wall, and tell which of your organs is inflamed by its elevated temperature.
Level 5: In space, Shelley can hear you scream.
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New cells and neural connections were necessary to expand the amount of information coming in, and accelerated transmission and interaction was needed to put it to good use. You could say that she's manipulating her nervous system a bit more generally, but specializes in this one angle because the first, involuntary burst wound up tossing her down that road.
In the experiment, the new cells and connections were built in the initial sparks of her awakening. In her discomfort, she wound up suppressing the ability, but that didn't make the new connections go away. The speed and reaction enhancement that lets her process all of the new channels simultaneously requires that she actually have her psychicness turned on, so until that happens again her sensory nervous system just seems like a tangled mess.
I freely admit that the rules of cool, plot, and drama played into it as well. Shelley relies on a lot of technobabble for what eventually boils down to a simple "keen senses" shtick, but I guess that's psionics for you.