I Am In My 23rd Contraceptive Pills And My Doctor

I am in my 23rd contraceptive pills and my doctor prescribed a once in a day Clarithromycin for 7 days for my ear infection. When I reached my 25th pill i finally have my menstruation as I expected and it only took 1 day which is not normal (my normal is 3-4 days) and it was gone after my 5th day of antibiotic and after that i have been experiencing cramps everyday until now. its been 3 days already. what is this i'm feeling?...