Purple (great handle, by the way), what you're describing is extraordinarily common. While I can't speak to what the root cause of the issue is, know that you're not alone.
It's often been said that the difference between success and failure lies less in motivation than it lies in determination. The old saw of, "1% inspiration, 99% perspiration" comes to mind.
Point is that successful folks will tend to slog through the difficult times even when their motivation has dried up and blown away. They continue to work away regardless of whether they're motivated to do so or not.
Another issue at play here is the reliance upon feelings, such as the feeling of being motivated. Feelings have an important role to play, but they're only one aspect of the whole dynamic. Feelings change, all the time, for reasons that may have nothing whatsoever to do with the task at hand. Hunger, exhaustion, physical discomfort and whatnot all have a profound impact on one's feelings. The key, I think, is not to ignore them, but to keep them in proper context; that is, to understand that the feelings you may be experiencing (ie: lack of motivation) aren't necessarily related to the task that you've set out for yourself. They're simply reflecting another aspect of your current situation (haven't eaten in a while, didn't sleep well, stubbed your toe on a table leg, etc.).