Developing a bruise after getting bit by a bug is from skin bleeding caused by a chemical the bug used while feeding, a stinger or a certain insect venom. When the skin bruises, it simply means that there has been a loss of oxygen to the skin cells, which is usually a result of a ruptured blood vessel beneath the surface.
Some insects, such as the common louse, chigoe fleas and bed bugs, are actually human parasites, which means that they feed on human blood. These pests latch onto or burrow into the skin as they breed and lay eggs. As these creepy crawlers derive nutrients at the host's expense, they sometimes leave bruises behind that swell on top of the skin.
As the wound begins to heal, the bruise often changes colors; this is part of the healing process and is totally normal. If the area around the bite is causing intense pain or abdominal cramps, see a doctor immediately in order to rule out complications from the bite. If the bruise around the bite increases in size and remains firm, it is possible that a hematoma has formed. A trip to the doctor may be required to drain it.