HELP US, PLEASE!!!

I truly hope that no one gets grossed out by this, but we are being overtaken by FLIES!! Just today alone, I have already killed 75 flies.. They are gathering at my bay window in the breakfast area and at my back door window!! Lucky for me they are gathering here because it is a HECK of a lot easier to kill them while they are on the windows.. I can not for the life of me figure out where they are coming from and why all of a sudden we are being invaded!! There are no odd smells to suggest that something is dead in my house and I can't find where they might be coming in from.. I honestly don't know what to do and CAN NOT stand it!! They are driving me bonkers!! If anyone has ANY suggestions as to how the heck to get rid of these PESTS please comment and let me know!! I would be happy to take any suggestions!! We are leaving for a long weekend on Friday and I am terrified that I am going to come home to a house FULL of flies and if that happens I will probably vomit at the site and never want to return to my home again!! My hubby is going to get some fly paper and attach it to the shades on the windows that they like to stay by, hopefully that will help ease the fly infestation while we are gone, but really I am at a loss here!! So again, any ideas would be totally welcomed :o)