I think I had a problem similar to this after I upgraded. I never got the overheating warning but my phone was very hot and my battery life was halved. This is because some app/service is keeping the processor chugging away. You can usually find the culprit by going into the settings and checking the battery usage (under More). It should be obvious if an app is going nuts. Usually my screen takes most of the power so if an app is using more than the screen, that is a red flag to me. Shutting down the app might fix it if it just got into a bad state, but more often I usually have to turn something off in the settings (like syncing or notifications) or remove the app completely.
Something to check into if you are still having problems.