Went to the physio on Tuesday. He said I'm pronating a lot (in both foot but especially left foot) and getting tendonitis in the tendon that helps support your arch. He's given me some calf stretches/rolling to do as well as some hip strengthening exercises (because my left knee is slightly knocked kneed).
It would be best if I get custom orthotics, but my health insurance doesn't cover them, so I'm going to try off the shelf insoles first (Soles) and get new running shoes because mine are quite old and worn down. Can't deal with getting custom orthotics until I'm in the same place for at least two weeks anyways.