breakthrough bleeding?

I started the pill in 2013 and stopped taking it a few months ago. I experienced breakthrough bleeding about a year or so into taking it. I started taking it again about a month ago. After two weeks back on the pill, I started bleeding again just like a period when I had my period two weeks prior. Why is this happening and what can I do to stop it? It's gotten to where I don't know when I will bleed so I have to be prepared at all times.