You two should expect the unexpected now, because that's what you're most likely to get. Any plans you make as a couple could be suddenly set off-course by something unexpected that comes up, much to your surprise -- and it may not be a welcome surprise. If you make a date to get together, one of you could get stuck in traffic and show up thirty minutes late; if you decide to cook dinner at home, the oven could malfunction and your meal gets overcooked.
And since things feel suddenly tense, you'll both have a hard time taking these little changes in plans in stride. But it doesn't have to be the stability of the relationship itself that's undermined, even if that's what it feels like is happening. Think of it more as the universe reminding you who's in charge. It's certainly not the two of you! Just because normally things run nicely along tracks that you can understand doesn't mean that a bit of spontaneity and unexpectedness shouldn't shake things up every now and again. So try not to get into silly little fights whenever things don't go as planned. Instead, loosen up a little, allow for mistakes and get ready to go with the flow for a week or so.