I having my 2nd period this month with only 12days gap from my 1st period. why is that?

Menstrual cycles can fluctuate from month to month depending on hormone fluctuation caused by stress, weight loss/gain, exercise, and lack of sleep. It is not usually a concern if this happens once or twice. If your cycles cause dramatically increased bleeding or you stop bleeding you may want to talk to you Dr about the situation and have a thorough exam and lab tests.

Probably it is not a normal menstrual period, but abnormal bleeding between periods. Or you may be developing irregular periods. Any and all abnormal vaginal bleeding should be professionally evaluated. Many causes are mild, but some are potentially serious. It isn't possible to predict the specific cause without a lot more information. See a doctor soon.