Thanks for your question.
In a women who was having regular monthly bleeds first possibility in the given situation is a pregnancy
. If you are in doubt due to faint line you may repeat the test after a week or else you may go for a trans-vaginal ultrasound
scan to differentiate between a late period or an early pregnancy. It is not seen in non-pregnant ladies even if they miss their period.
Safe period is not a very effective method of contraception
& failure rate is considerably high. Aggressive dieting & exercise may effect menstrual pattern but this must be a diagnosis of exclusion.
Please make certain of your pregnancy status as certain diets & exercise plans may harm your pregnancy.