My period was 4 days late and I took a home test the nest day it was neg. I threw up last night and I had sex went to the bathroom and I saw a spot on the rag so is this my period

It . It is very difficult to determine if your period has started based on one spot of blood. Your menses (period) may start with light spotting and eventually progress to a heavier flow, sex can frequently get the flow started so to speak if your menses due. Based on the date of this post, you may have already begun your regular menstrual bleeding, if not i recommend you take a home pregnancy test or have one done in your doctor's office. Good luck.