Yup. One word for you: hormones. Hormones are stored by your body in your body fat. When you suddenly shed a lot of that body fat, the hormones are also released from storage and can throw things off, especially for women, until weight loss slows down a bit.
A visit to the clinic is still not a bad idea to rule out any problems or pregnancy, especially if you have been sexually active without birth control. Even if the pregnancy test is negative, a quick pelvic exam can often confirm if you are pregnant or not due to changes in the appearance of your cervix when you are pregnant.
Hope all turns out well!