You can read more about bed bug bites and their symptoms here
Fecal matter is the excrement that the bed bugs leave behind after digesting the blood meal that has been drawn from the host (you). Bed bug fecal matter looks like a small black dot and consists of digested blood. A bed bug will usually deposit one dot of fecal matter after a feeding. This information is useful in determining the approximate length of time that a bed bug infestation has been present in a given area. It is possible to reverse engineer the math when counting the fecal matter dots in determining the approximate length of time that the bed bugs have been present.
Bed bugs are exoskeletal insects meaning that, unlike humans where our skin grows with us as we grow, bed bugs shed their skin for each stage of growth that they go through. These exoskeletons are signs that bed bugs are in a particular area. Finding bed bug cast skins could be indicative of bed bugs being present although a cast skin could be from a previous infestation as well. Cast skins have been shown to exacerbate symptoms of asthma in children when they are exposed to many in a confined space and are inhaled as they become airborne.