I'm not a pharmacist or doctor (nor did I stay at a Holiday Inn Express last night), and I'm also no lady
, but as someone who worked in clinical pharmacy for 15+ years two things come to mind. The first is that one of the ingredients in Midol (or generic equivalence) is pyrilamine maleate. This is an antihistamine and can cause drowsiness (and that loopy/spacey feeling). One of the other ingredients is caffeine, so drinking coffee on top of the caffeine in the Midol could give you unpleasant side effects as well.
The idea of the 3 ingredients in Midol are to reduce pain and reduce bloating. The acetaminophen/caffeine combination is an effective analgesic and the pyrilamine maleate (as with most antihistamines) acts as a minor diuretic. I wouldn't consider Midol a mood altering drug in the pure sense of it like with opiates or benzos, but drowsiness/dizziness is a common side-effect from most antihistamines.
I'll throw in the disclaimer that this is not medical advice, but merely an explanation of how the ingredients in Midol work. All antihistamines can cause drowsiness. Even the newer ones that purport not to cause drowsiness still list drowsiness as a side-effect.