Mine was not at San Antonio but I had this same experience like you, took a day off work, was so ready and tensed at the same time. A couple of hours before the interview I got a call that my interview was rescheduled, I immediately thought this was a scam, I started calling all the numbers I could call, even called USCIS just to speak to a tier 2 officer who told me that it was due to my file not getting to my interview location on time, they were kind enough to give me a number for the location where my file was, I called and they were kind enough to tell me they had sent my file and it could take a few days. I ended up having my interview in December after almost 3 months or so from the original date. Do not panic, keep preparing, they will definitely send another interview notice atleast 4 weeks in advance, don’t worry they won’t surprise you with a two days notice. You are almost at the end of the tunnel, just keep your head up.