Good/normal peak flow but feel short of breath?

My asthma is generally well controlled with Symbicort 200/6, which I take 2 doses in the morning, 1 at night.

This past week or so I've had this recurring feeling of not fully being able to catch my breath, I think the correct term is 'breath satiety' I feel like I can't inhale the maximum I should to feel satisfied, it presents itself as a feeling of tightness in my lower chest. It doesn't impede me, I work as a postman so I'm very active, it's more of an annoying sensation. Strangely when I'm exercising the feeling abates, it only really presents itself when I'm sat down/idle.

Obviously I'm at the point of going to the GP, but I thought I'd post to see if anybody experiences similar.