I'm a doctoral student in audiology and while I'm no expert yet, have done a number of evaluations of dizzy patients (we deal with the whole inner ear system--so that includes balance in addition to hearing). Short episodes of true vertigo (spinning) associated with getting in or out of bed are hallmarks of BPPV (see the link CycleTherapy posted), and I suppose getting really jostled around on a MTB could contribute. Did the spins go away within a few minutes each time?

Regardless, catching up on your hydration and nutrition seems like a very good place to start. Let us know how it goes!