Last month my periods were 4 Days early and they ended in 4 days instead of the usual 6-7 days. I have revently started using tampons. Then on the 6th day I had sex and used condoms the whole time. But while having sex I started bleeding from my vagina again. We stopped having sex, the condom was thrown away and it was intact. Then I kept bleeding for 3 days. Then I didn't have sex for the remainder of the month, just made out about 4 times with no vagina-penis contact.
A month after my periods started I had 3 days of partying so I consumed a good amount of alcohol but my periods were no where to be seen. Now my periods are 4 days late and I'm still debating whether I should take a pregnancy test.
I would like to know why my periods are delayed as I did not have sex. And what else could be the reason.

Weight gain or loss as well as stress can cause a late or missed period.

Pregnancy is still a possibility. Wait until your period is two weeks late then test using first morning urine.