Почему в 15 лет нет менструации, сыпи и боли?

Having some variation in cycle length from month to month is pretty normal, and it's also normal for cycles to range between 21 to 35 days. In your case having cycle lengths of 23 days and 28 days is within the normal range.. And pain and cramps during a 28-day cycle is quite normal which almost all women go through. If its really unbearable you can condult your doctor

Light red spotting after not getting a period for one month may indicate implantation bleeding, hormonal imbalance problems or complications to the pregnancy itself. It is advisable to see a gynecologist for proper medical checkup and also receive an accurate diagnosis. 

When your menstrual period is 20 days late, and you have such symptoms as gaspiness, nauseousness, headache, haemorrhagic pastula – it’s high time yo seek a gynecologist. The negative pregnancy test result together with the IUD you have may signify that there is an underlying medical condition which needs to be treated. You should visit a gynecologist specialist to receive the right treatment and get the correct diagnosis.

irregular periods are common and can be caused by stress . A healthy diet and exercise can help regulate periods . hormonal imbalances or certain medications can also affect periods .If you are concerned, speak to your DOCTOR.