dozens of flies in the cat room?

So this may be a  bit weird, but I can home today and there were DOZENS of flies in the room where we keep our rescue cat and her 4 kittens. We got all five cats 5 weeks ago when the queen had the kittens on our doorstep ( literally) and we generally keep them in one room (read: we keep the litter boxes, food and bed in the same room). The kitties just started eating food this week and thus using the litter box. we have had a little trouble with peeing everywhere but what can you expect 4 days into litter box training? Today there were actually no accidents! I very happily cleaned their little litter box rather than mopping the whole floor  The weird thing though was that there were 3 to 4 dozen flies in the room. Now, we have had a few (like 5 or 10 at most at one time) in there since we got the cats but between the food and the litter box and the window (with a screen) we expected them. This sudden invasion though has me confused/concerned. Does any one have any explanation for my sudden swarm of flies? Could this be an indicator of a health issue or is it just a result of the pee I 'thought' I was cleaning up.

 Thanks All!

The first few flies may have had babies.  If you have a black light, you can find pee spots easier.  They will glow in the dark under the black light.

It isn't hard to miss some spots.  I really thought I was ahead of the mess until one day I discovered mold under the tray where I keep one of our fountains.  Now the tray gets thoroughly washed at the same time I do the fountains - once a week.  I used to just wipe it down every so often.

That's weird!  I wish I had some words of advice for you.
 

I had this happen to me once and I found I had a cat carrier in the room where the flies were and did not realize one of my cats had thrown up in the back of the carrier under the blanket I had in there. So I am guessing it formed maggots I think they turn into flies. Gross but there was so many flies. We found one time in the couch when I was a kid dog milk bone fell into the cushion and when I went to throw the bone out it had formed maggots. It was hot out when the flies were around maybe that had something to do with it.