From your history you don't seem to be pregnant.
It's very good you have taken extra precaution by using condoms. Luteal phase varies from person to person. Yes it is definitely affected by stress both physical and mental. You having exams, your luteal phase must be affected. It's absolutely common, but since you are worried. You can do a urine pregnancy test for precaution.
Your periods will regularize after 1 or two cycles. You can try doing mediation or yoga. This will help you with your stress and menstrual cycle.