I just had my period on august 23 and its back on again today september 6th. why is this happening? i started back on my pills that following sunday on august 30th. i have been taken cold medicine and antibiotics.

Breakthrough bleed: Breakthrough bleeding (bleeding at the wrong time) is very common on birth control. It happens in about 1/3 of users. Its no big deal and usually self corrects over time. If you are not satisfied or it continues to happen then consider using a different birth control method. It is a common side effect and does not mean something is wrong or that the pill is not working.