I would go back to the doctor and let them know. I would not wait if you think it is blood.
Hm. Well, first---- antibiotics don't work on the flu as that is a virus. People often get secondary infections post flu though.
Anyway, rust colored phlegm could be the result of old blood down in the lungs if you are coughing it up. Typically it is nothing to worry about but once in a blue moon it could be tuberculosis which requires medical attention. Doubtful that you have TB but you could check with your doctor to be on the safe side.
For phlegm and mucous, it helps to double your intake of liquid to keep it loose and flowing. Try not to use a cough suppressent as it is better to have it moving. I also think mucinex does a good job keeping it thinner. (over the counter).
I don't know if that helped but do hope you feel better soon. goodluck