Im not sure if it occurs this quickly (ignorance on my part) but this is what people call the test flu. You will feel like you have the flu, and will feel tired, sick, etc. Not everybody gets it, and you may not even have it - you could be actually sick, but your illness coincided with your first test shot.
If its the test flu, you'll be fine in a day or two. My friend kept experiencing it during his 1st cycle, while I never felt it.