I take contraception and an irregular bleeding started to appear?

I took Contracep a few months ago, I did the second injection, until then everything was in perfect condition. This week a bleeding bleeding began to appear. It's normal? A few days ago, how long must I wait before looking for a doctor?
Expert answer:

Irregular bleeding is very common with the use of Contraceptive, seek medical attention if bleeding does not stop or begins to recur many times.