I understand your concerns but there s no reason to be worried. Since you have had your period quite regularly till now one month of periods being skipped indicates no serious problem. There are various reasons why the periods maybe irregular, the most common being due to emotional stress. The other factors that cause this could be hormonal imbalance, eating disorder, increased exercises, Hypothyroidism, uterine scarring, use of contraceptives, breast-feeding, and medications. Smoking does not affect the menstrual cycle. As you have already taken the home pregnancy test and the results are negative there is nothing to worry about. I would suggest you to wait till the next month of menstrual cycle and if it continues to be irregular then you could consult your obg-gyn doctor.