She can't feel her throat, she can't feel her knees
She’s giving him “head,” on her knees, presumably for so long she can’t feel both her knees and her throat. This line is probably also referring to being numb from all the drugs they are taking.
“She can’t feel her throat” may mean she’s snorting coke and she’s feeling the drip, or that she’s been singing along/screamin at his show so loud that she’s sore
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