I agree with the others that talking with your doctor about this ASAP is important.

The therapeutic range for lithium is quite narrow (as a guide, blood levels between 0.6 and 1.2 mmol/L) - levels lower than this may not have much effect and higher levels can have undesirable side-effects.

Your doctor should be able to arrange for a test of your blood lithium level and (depending on the results) adjust your dose up or down to keep it in therapeutic range.