I used to be more physically fit and would work out daily, non stop. I loved it. Life happened, lock down, etc and gained a TON of weight. Decided to get back into things and start running/working out again. 
 I run/brisk walk a 10k every morning. Never had a problem. Well, the weather is starting to get cold and I didn’t want to lose focus, so I joined a gym. I still do my 10k everyday outside since the weather is nice and then I go to the gym and do some cardio on the bike and lift weights.
 I noticed that when I do my 10k outside I never have any problems what so ever and I feel great after. Last week it was raining so I went to the gym to do my running on the treadmill and I made it maybe 0.3 miles before I got a horrible shin splint. Never had one before either.
 I took a day off and did my 10k outside again and don’t have an problems, no pain. Later that day went to the gym and went on the treadmill and BAM, shin was hurting bad right away. I have done this everyday for the past week and same thing. I can run outside fine, but the treadmill just hurts my shin so bad.
 I am not sure what to do. I don’t want this to be a problem when the snow hits and I can’t do my outside running anymore. What should I do? Does anyone have any advice?

Things I’ve tried: - stretching ( I even used NTC runner stretching videos) -taking time off - new shoes - pay attention to my form and keep it the same as when I am outside or atleast close to it.

I am at a loss