How is it that we used 13 flea bombs on Sunday and still have fleas? Have only 1 INDOOR cat...?

We have what appears to be a mive flea infestation. We have one cat who is strictly indoors (has never even set foot outside). Baffled as to how we have such a problem. This past Sunday, we bombed our home...in fact, we used 13 (yes 13! flea bombs!-and live in a duplex our home is small). To our dismay, we still have an infestation. Please help! I'm truly losing my mind. (and my blood as the fleas seem to adore me)