
I run my dogs in long grass from time to time and they have had a lot of grass seeds (some of which are VERY small) stuck in their fur. I prefer to walk after rain as I'm sure one of mine inhaled something, which made her very distressed a couple of weeks ago.
I'd hazard a guess that it's more likely to be the seeds causing the irritation than flies etc. Especially right now as everything seems to be going to seed.