It's grass from July to august. I am allergis to grass pollen it would seem and having the same breathing problem. I make it worse by thinking about it and then I end up causing a minor panick attack which does cause shortness of breath.
You could have hayfever, might be anemic or just simply tired. Also I suffer from acid reflux and that can cause shortness of breath too.
If it continues, best see gp. You might fine in a month or so it goes, if it does then I'd say it's hayfever (grass pollen allergy).x