Lantus notoriously can cause lows.
But I also want to say steroids can be an issue. I really really react to them, they work great but even a small amount and my BG level shoots up and requires a lot more insulin. I avoid them as much as possible but sometimes they are needed. Last time I was given a low dose shot for my back I used 16 extra units that day with hardly eating. I had to take pills for a few days and required extra insulin each day. And they stay stubbornly high for a few days plus after. But the worse is all of a sudden I go back to normal and then I will drop, and I don't know when that is going to happen. I know this from experience so I watch for it, but it's all of a sudden with me. I have to wonder since you said you had just returned to normal the day before if that might have happened to you.