Hi!!!!Each individual's body reacts to the drug in a different manner.If you are developing bruises and petechiae
,immediately stop Doxycycline as well as other drugs if you are taking any.
It is reported that some patients may develop rashes after its intake(as one of its adverse effects).I would suggest you to see your concerned doctor so that he/she will be able to examine you and rule out drug allergies
if you have developed any.
Some precautions should be strictly followed while one is on Doxycycline like alcholol should be strictly avoided.