I think running down hill is one of the big contributors as your foot is flexed down (plantar-flexed) resulting in more stress on the shin muscles/tibia. If the profile of your runs has changed suddenly from flat to hilly, that could be a likely cause.
Ideally find a flatter route, and rebuild the downhill mileage slowly. Easier said than done in some locations!. Also, are you running on harder surfaces now compared to before