Well, look at it this way. This is the same thing that happened last cycle. So this is probably just your body's temp pattern ... we're all different. If mine was that way I'd consider it LUCKY. You won't be thinking you're pregnant every cycle, because you think your chart looks triphasic, testing every day and getting BFNs, and then having a sudden temp drop and then AF. You will know you're not and that's that. If your temp keeps going up it's incredibly stressful to wonder "Why am I still getting BFNs? Is there something wrong with me? Is there an embryo, but it's miscarrying so my temp hasn't dropped yet?" etc. You just know you're not.
That being said, we're talking about a drop from a ONE-DAY spike. If your temp hadn't been high a couple days ago, this "drop" wouldn't be upsetting at all, would it? It would just be a normal post-O temp, on the same level as temps from the whole week before. Nothing weird about that, and no reason you can't be pregnant with that. Not all pregnancy charts are triphasic.
Think of it either of those ways, whichever you prefer, and maybe you'll feel better.