commonly occurs when ejaculation takes place after penetration in the ovulatory phase. But in rare cases it has occurred in situations like yours. When ejaculation occurs in the time when you are ovulating ( egg released) the cervical mucous of the women gets thinner and more sticky and favourable for the motile sperms to find their way into the vagina. The sperms are alive for 72 hours and conception is most likely to occur.
When you are in the ovulatory phase ( 11th -21 day if you have a 28-32 day cycle on an average ) you should use a barrier contraceptive
if you are not on pills. If at all un-protected intercourse
takes place, it is better to consult your gynecologist immediately and take emergency contraception
within 72 hours.
I will suggest you to visit a gynecologist have a test for urine ( pregnancy test ) and if required also get some routine blood tests done. It is important for you to get examined clinically, your uterus
needs to get examined. Do not wait till it is too late. There is nothing to be scared of, you will get fine soon. Take care.