There are a number of reasons for your irregular bleeding. You can read about some of them in this article written by Merely Me, a writer and community leader for this site, titled, Bleeding In Between Periods: What Does It Mean?

You can take a home pregnancy test if your normal period is more than a week late. It's unlikely that you are pregnant if there was no genital contact without the condom in use. Anything is possible but chances are small.

Is the weight loss due to your worry, or had that started prior to this? You need to be concerned about such a rapid weight loss, if it has been. A doctor can also test you for pregnancy if you wish, and look into any other health problem or symptoms you are having. I hope that article helps.