What Causes Delayed Periods?

So my period is usually on time like clockwork, every 28 days I start, and then the same for the next month. But I was on the nuva ring for almost a year and decided to stop using it. No reason, just couldn t afford to get more. But ever since I stopped I was having very irregular periods for about three months and now it has been back to the normal clockwork. But my boyfriend and I had sex about a week or two after my period ended in the end of January and I still haven t started. Could I be pregnant?