Given the negative pregnancy test you are probably not pregnant. That spotting and period delay are probably due to using emergency contraception (plan B). It is impossible to say when your next period will come because your cycle was completely disturbed by the additional hormones from the emergency contraception. You can only wait to establish your regular periods spontaneously (when ever that happens) and to use a condom for contraception. Because of this cycle disturbance emergency contraception mustn’t be used regularly.