Running spells which can be cancelled are shown on the left bottom side of the main bar below the portrait, very tiny round symbols (if you are on PC). But concentration on spells does not hinder you to cast other spells, also not other concentration spells (you just put the new spell in place of the old). So it must be something else.
If it is armor, it's shown in a yellow warning message that there is no proficiency with it if you hover over the armor piece. I do not know any boots or gloves who need heavy armor proficiency, and the only helmet that needs it to my knowledge is the helmet from Grym, so most probably it would be body armor that caused proficiency problems.