I'm 20 yrs old female 

I suffer from moderate to severe acne since my teenage years but in the last few months it has gotten worse, last week I went to Dermatologist he gave me an Antibiotics (oral Doxycycline & topical Clindamycin) as he said my acne are cystic.

Basically until yesterday there is no much difference to mention except that the redness of the pimples has reduced little bit , but yesterday after I had my Doxycycline tablet I felt very nauseated and unfortunately I vomited, I then washed my face and there was the shock My pimples became all dark purple color as if blood is just about to explode from them 

I am so worried and scared and it makes feel very uncomfortable about my look :(

why did they change into that purple color?

Is it something dangerous ?

ps: I don't know if it has any thing to do with that but Yesterday was the second day of my menstrual cycle.