Thanks for posting your query.
In IUI, actively motile & washed sperms are inserted inside the uterus
during ovulatory period.
However if every things are normal like tubal patency, adequate amount of actively motile sperms, still there are only 15-20 % chances of conception in one cycle of IUI.
So do not worry,you are having adequate follicular growth for ovulation & it is only one follicle that is fertilised in last.If two ovum are released simultaneously, chances of twin pregnancy
Chances of fertilization increases, if IUI is done with in 3-4 hours of ovulation & if IUI fails you can expect your period as was expected in previous cycles.
I hope your query has been answered.