I was taking lithium 500mg a day for 20+ days but when i checked my blood lvls it came back - 0.3 mmol/l (range: 0.3 - 1.3). is it my fast metabolism?

Various reasons: The result you received could be based on a number of factors. It depends on the total amount of Lithium taken.Other medications can impact the level. Kidney function status, along with hydration status, play a role. Timing of the Lithium level obtained also matters. It is important that you get your level drawn within 12 hours after the last dose to get the most reliable result. Hope this helps.