I've been getting yeast infections about every 2-3 weeks since getting the mirena (levonorgestrel) inserted. Could the mirena (levonorgestrel) be causing them?

Possibly. Having an iud can conceivably change the environment of the vagina enough to encourage yeast overgrowth. Avoid other things that encourage yeast such as sweets, lots of carbs or alcohol. Or wearing underwear or clothes that aren't breathable.
Confirmation. Mirena (levonorgestrel) is not associated with recurrent yeast infections. The next step is testing with your doctor to confirm that the infection is yeast and to confirm that the medication you are using is the correct choice.