What could be the reason for getting random bad brusing on legs ?

please answer quickly im really worried random bad brusing on legs i am 23 and and i am a single mother of two children i am about 5 0 137 pounds, my four year old son was just recovered from wilms tumor cancer and now this .... any ideas?i have very badly bruised legs with no known cause for the bruises. they at this point are just popping up on thier own leavingt very very large dark bruises almost covering both of my legs. i notice that the bruises have a red tint at times torwrd the top of the skin but only in the darkest and biggest ones. since lunch today alone roughly 5 more have appeared. i have had a nlittle back pain and leg cramps at night more then anything. what could this be?