I called the clinic today, and they talked to the doctor and confirmed that it should have been 500mg, not 250mg. They phoned in a prescription for a weeks worth of 500mg pills, and I verified that they did it correctly by accessing the clinic's "mychart" app.
Sorry to bother everyone with this hypothetical (of what if the dose change was intentional, and they didn't even bother to try to tell me, but left it to be a surprise when I picked it up at the pharmacy). It's just that I couldn't believe they made the same mistake twice (twice in the last 3 times), so I figured there was a good chance it was intentional.
AND because in the past they did do something similar like this -- prescribed the same medication even after I said I had tried it and that it didn't work. As if I was some dumbass that can't follow directions, and to "try again". (I ended up trying it again anyway, and again, it had no effect. I'm very meticulous too about following the regimen)
Also pissed about that "Ann" who last time phoned to tell me that the prescription had been sent to the pharmacy, and who, incorrectly said yes when I asked, "is it the same as the previous prescription -- 500mg of Cephalexin?". Just too busy to check, busy busy you know. Easier, quicker, and more "efficient" to just say yes.
Anyway, thanks for all the input. I'm happy to hear that its apparently not the practice of changing prescription dosages without telling the patient (becuz that would take too much time, and office visits must be kept to 10 minutes and that sort of thing, and phone calls to a minute). I have a very negative view of the medical profession.