Jeff, repeat the CYA test. I doubt that the CYA dropped like that as the only way to reduce CYA is a water exchange (quickly) and (slowly) during season due to the aforementioned rain, splash out, water fills (so I have been told). Even 2" of rain would not reduce CYA and I would know as I had over 2" of rain dumped into my pool and my CYA hasn't moved from 65. Now the best thing to do is pull the stabilizer from the pool because if you were at 40 you were good there depending on your environmental conditions. Now, you may want to keep your FC using bleach or liquid chlorine at the high end of the recommended levels for CYA of 40 (7ppm). Because in a week you may see your CYA raise due to however much stabilizer dissolved since yesterday afternoon, maybe not much.
Now it could be you had old reagents or the new reagent is bad, or you tested incorrectly this test or the last few tests. Here's where you have to trust your gut. Did you dose your pool for CYA of 40?? Then you are probably at 40 because of the simple fact that CYA, as mentioned, is a long term stable chemical in your pool. It's a necessary evil, Jeckyll & Hyde. My CYA is 65 because I just got into the BBB Method last year after using pucks (constantly adding CYA) for three seasons prior.
The best thing to do is not overreact to tests that are "off" EXCEPT for FC which you should attack with a vengeance if it's below recommended levels in relation to CYA. Trust me, we are all guilty of it because we are the type of people who want concrete numbers and exact science, hence why we are here using a "calculator" to adjust our pool chemicals. Yeah, people may call me crazy, and I would have thought the same if I heard someone was using a "calculator" for their pool, but my pool is so freakin' clear it amazes anyone who sees it. And for that the naysayers can kiss me arse!!
Hope this helps!