Thanks for the query.
As you had spotting in the previous IUI attempt also, this time better to consult gynecologist once.
And as you crossed your expected date of periods also once go for pregnancy
If the test comes positive the spotting you had could be implantation
Start using progesterone
pills with your doctor's advice.
And if the test comes negative, the spotting may be indicating failure of IUI.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429