what position is your head in when it happens. . . streight back, to the side, or whathave you.

also, as with any physical exertion, are you eating and drinking properly? either of those could cause the same symptoms

Along with the other possibilities mentioned, yes, it could well be your inner ear not being used to such positions/movements.

Try doing slow and deliberate movement on the ground, rolling from side to side and completely over; doing slow sommersaults; headstands, etc. The idea is to get your ear-stones used to being moved about. Also try moving your your head more slowly when you balance -- don't slam your head about to retain or regain a balance.

If it doesn't go away, or gets worse, or just kinda lingers there making free movement difficult, you need to look up Dr. Wobig.

Wobig has an office on Stark near SE 110th (or so). The sign outside is for an eye clinic. Wobig has a chair that moves you about in three dimensions, that he's programmed so that it puts the little stones back where they're supposed to be. (You can sometimes get the same result by tilting your head to odd positions and gently shaking it. Sometimes.)

He had a write-up in the Oregonian about a year ago. Should be Googleable.