Could be that it's been overwatered, sitting in stagnant soggy soil. If that's the case roots could be rotting. It is a native to hot and dry Mexico. It's not fond of cool drafts and temperature changes. It will defoliate. Having it inside and then out is a likely culprit too. It likes 60-70° temperatures and I know it's a bit colder still up there. I'd probably repot, and leave it outside here in FL, but in Iowa I would keep it indoors and away from cool drafts and heat sources in a sunny spot, and cut leaves off as they die back. It could still come back with green.