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You had unprotected sex just after your period ended, you should understand this is safe period as ovulation takes place in middle of cycle approximately 14 days prior to expected date.
You took emergency contraceptive pill
which is progestin hormonal
pills and cause withdrawal bleeding and some associated nausea
,pain in abdomen.
You should see a Gynecologist to examine you internally to see everything is fine your symptoms could also result from some infection if so it may be treate dwith proper antibiotics.
If your bleeding stops and is due to hormone only it should pass in few days. In future please practice safe sex
or you may choose any regular contraceptive method there are many from which to choose.
Hope this information is useful to you.
Dr.Akhilesh Dubey M.D.