What Causes Dark Spots On The Skin After Bitten By Bed Bugs?

hi i think i hate bed bites/dust mites i have found blood spots on my sheets and i used to feel (thought i was imagining) little bites from time to time when sleeping. over the past month i have been getting darker tan spots on my tan skin and they arent stopping i am going to see my dermatoligist tomorrow but im very concerned is this a reaction to dustmites or bed bugs/fleas? i seem to wake up with more everyday?