Why does my skin always do this when I get bit?

Every time I get a mosquito or other bug bite, it turns purple. It usually happens the day after I get said bite. They usually don't get bigger but they ALWAYS turn purple and sometimes get kinda splotchy like a bruise or hickie. Why does this happen? They don't hurt or anything, just turn to purple and itch like crazy but of course its a bug bite so it always itches