morning after pill can cause side effects like irregular periods
it will depend on the time of the menstrual cycle when you take the pill,
if you took the pill during follicular phase
means before ovulation you may have early onset of periods.
so that may be responsible for your bleeding,
anyhow if you do not get regular bleeding at expected date better to check for pregnancy
once to rule out implantation
bleeding etc as there is 1% failure rate for the pill.