if you're really worried that you're pregnant, then take a pregnancy test. It will put your mind at ease either way: if you are, then cross that bridge when you come to it. I'd take it as soon as possible though, because tis too much of a major issue to have hanging over you!
Whatever happens though, I'd take yourself off the pill you're currently on. Although you say one of the side effects is no periods at all, losing half a stone isnt exactly good for you! Was there a particular reason that you switched in the first place, such as a medical reason? If there was, go to your doctor, and ask their advice.