Something went seriously wrong, like a drug overdose, during a "strange encounter" (aka, an encounter with a stranger/casual hookup), where the narrator suddenly realizes that the drugs and casual sex is "free" and "not cheap" but they were "giving [themselves] away." He was trying not to "freak out" about a stranger in desperate need in his house ("almost died in my house"), then "the moment [she] came to [he] swore [he] would change", i.e., would not treat such serious topics so casually.
Something went seriously wrong, like a drug overdose, during a "strange encounter" (aka, an encounter with a stranger/casual hookup), where the narrator suddenly realizes that the drugs and casual sex is "free" and "not cheap" but they were "giving [themselves] away." He was trying not to "freak out" about a stranger in desperate need in his house ("almost died in my house"), then "the moment [she] came to [he] swore [he] would change", i.e., would not treat such serious topics so casually.