Lower back and shin pain when running?

Hello. The question is actually already in the title.

For several years now, I've been experiencing severe pain in my shins while running. This pain has worsened over time and has become more frequent. Currently, I can barely run for more than five minutes without it starting to hurt. Now, it's also radiating down to the lower part of my back, and every step is painful. After a short break, it's better again.

When I exercise (at the gym), this doesn't happen unless I'm on the treadmill. Also, the heels of my shoes always wear down quite a bit, sometimes at an angle, so I suspected it might be due to my walking style.

What could be causing this? Should I see an orthopedist, and what can I do about it?

Thank you in advance and best regards!