Hello. Our hormones can shift and our periods can change. It happens and will continue to happen throughout your adult life. It is really unlikely you got pregnant with this but since you did allow him inside your vagina without a condom, that is a risk. A penis emits small bits of sperm in these instances. It 'can' be enough. Again, I do really doubt it but if it happened 2.5 weeks ago, you can take a test to be sure. If you continue to have irregular periods, see your doctor for a routine pelvic exam. It might be a good time to consider the pill then as it will not only protect you from pregnancy but regulate your period. good luck