From your description they really don't sound like fleas and more like the thunderbugs that someone else suggested. From my (admittedly limited experience as I try to make sure that my pets don't have fleas in the first place
), fleas are about 3 or four millimetres and sort of rounded. Thunderbugs are much smaller and skinny. I've also never seen a whole load of fleas together or crawling, they tend to be on their own and they jump. If you had a lot of them and they were crawly, I really think they are thunderbugs.
I was always told that thunderbugs appeared in the summer when there was low pressure (i.e. like you get if a thunderstorm is about) because the pressure pushes them to a lower altitude. I have absolutely no idea if that is right or a load of rubbish though! it may just have been a convenient old wives tale! Anyway, good luck!
