I have no idea what it could be. All but one ADHD drug could do it, like Adderall, Focalin, Ritalin, Concerta, etc. There's also Provigil - a anti-hypersomnia drug that's being used a lot for shift-work fatigue, etc. Meridia - a weight loss drug could do it.
Of course, you would know if you took those. Let us know how your second test comes out.