Economics as a discipline didn't really exist yet, but its roots were there in the form of guys like Thomas Aquinas, Duns Scotus, and Ibn Khaldun.

That said, inflation and deflation both affected the economies of medieval states. I'm not an economist by any stretch, but there are many references to states "debasing" their currency--that is, reducing the amount of the valuable metal (gold, silver, etc.) in coins. The effect is to increase the number of coins at the expense of their individual worth. Often this was done when the king (or other ruler) needed a cash injection to either a) stop deflation or b) quickly pay for something big, like a major military operation.

Alone, debasement doesn't necessarily cause inflation, but if the king spends a lot of this new currency in a short amount of time--such as on a large construction project or preparations for invading a neighbor--it can cause inflation.