Gabapentin worked for me 3 days ago, now it does nothing at all at any dose. can tolerance build that quickly? how do i reverse tolerance?

No: Gabapentin is not prone to tolerance (aka tachyphylaxis). Usually the dose builds up in the system until at a therapeutic level. Once there it will not stop working abruptly (unless there is an interaction with another medication. If stopped it should be tapered slowly under your doctor's supervision. Stopping the medication for a time is the only way to reverse medication tolerance.