Hi and thanks for the query, Its important that you did a pregnancy test and it came out negative. Possibilities of getting pregnant with emergency contraception are actually very low, as in your case, less than 2%, but still possible. You might need to repeat the pregnancy test to be very sure, However, it is not unusual to experience a change in your normal menstrual cycle. Remember that the pills that you take are hormones. These hormones actually interfere with normal hormonal cycle of the body, and changes in the various concentrations could alter your bleeding patterns, which are of course hormone dependent. If symptoms do persist or any specific worries, consulting your gynecologist for assessment and management could be very helpful Thanks and best regards, Luchuo, MD.