I've had this before on previous rounds. For me it was food boredom essentially. I just had no appetite. I've found that trying a few new recipes that look exciting, or even having a favorite meal Whole 30 style (for me it's something taco related - taco salad or something I call a potaco which is a potato loaded with taco topings) makes a huge difference. Also if you've been eating the same foods for each meal, try varying it a bit. This will pass as you get used to and start looking forward to various Whole 30 meals.
It may also be that when you are eating quality and nutritious food, you just need less than you are used to needing. Or sometimes hormone fluctuations can cause a few days of disinterest in food.