The earliest you can get field mushrooms, is june. So it is not too much out of season but I have never found them quite this early. But i have found quite a few very recently that should be out july/august, so i do feel the season is a bit out. April was hot and dry and may was really wet so that weather was the wrong way around for those two months, so it is not surprising that things are a bit out. Mushrooms need high temperatures to heat the ground followed by rain which why there so many in september which may also explain why you are seeing a large crop of them now.
I stopped using season as a reliable for IDing fungi last year when I ate a giant puffball in november, when they are normally seen around now. Most of species i have seen this year have been 2-3 months early.