If you have regular 28-30 day cycles and have not missed a cycle, then the other likely possibility of the post coital spotting could be some local cause like vaginitis
or cervical erosion or polyp. If you have missed a cycle, you should test for pregnancy
at once. Please see a gynecologist for a local exam to exclude other causes also. A trans-vaginal/pelvic sonogram helps in excluding other pelvic causes. Hope your query has been answered. Take care.