Swarm of flies in the garden

I dont no if any1 can help me with my question? or if this is what im ment 2b talking about on here? i just need some help i have millions of flies above my grass in my back garden and i dont no y they r there it looks discusting!!! my children want 2 play outside but there is just 2 many out there and they r just flying every where, does any1 no y they mite b there or have any idea how 2 get rid of them??? (i have 2 say i have a dog that goes 2 the toilet out there but there has never been a problem like this b4! my boyfriend did cut the grass about a week ago and put grass feed down but 2day is the only day that the flies have been around, i just want 2 get rid of them!!