Yes, it's possible to conceive before a real period, assuming you ovulate.
Generally, a menstrual cycle (bleeding) happens only after you ovulate. If you fail to ovulate (it's often delayed after being on the Pill), you can have some dysfunctional bleeding, which could look like what you had. Give it some time. If you're not cycling normally in another month, check with your healthcare provider. I usually give such disorder a good three months to straighten out.