It could be from anything to be honest; one day is not enough to say its effectiveness varies from day to day. Things change every day and your body will adjust regardless of what supplements you take in. If you have more stress going on that day, if you got less sleep that night, if you drank a different amount of water that day, if you ate a different amount of carbs that day, etc. will all greatly alter the effectiveness of any product for that matter.
Plus remember, the first day on any supplement is always the best!!! Let us know how it goes after you use it a few times so then we can better assess the situation as I would be glad to help out; just going off of 2 days is a little short to piece problems together and solve them.