My period started again after 21 days, is it...

I got my period at the day 21th , is that ok ?
normal period interval is 21 - 35 days , any regular period in this range considered normal , but if you suddenly have short period interval after is been longer ( ex: 28 days ) , then its usually caused by one of those :
1. life style ( usually its temporary and period back to normal soon and usually there is no accompanied severe symptoms 0
2. stress 
3. medications side effect 
4. ovarian cyst 
5. abnormal productive system hormone 
6. thyroid hormone abnormalites 
7. uterine polyps 
Treatment of short period interval :
1. consult a GYN doctor to do the needed medical checks and determine the treatment pathways if needed 
2. avoid anxeity and stress 
3. avoid smoking 
4. make sure that your not taking any medication that may effect your productive system 
5. imporve your life style : eat well , exercise , sleep well 
6. in case of medical illness which may be abnormal hormonal levels , doctor may give you some medication to supress or increase productive system hormones such as primlot 
7. in case of medical illness such as abonormal thyroid hormone levels , doctor will give a thyroid medications such as thyorxine 
8. in case of ovarian cysts or uterine polyps , doctor may observe the condition with recommanding OCP , or may do surgery if the cysts or polyps presist or it is in large sizes 
9. in case of chronic disease  , doctors may do further medical examnation to determine the cause and the treatment 
Needed medical examnation :
1. physical examnation by doctor 
2. blood and blood chemistry test 
3. viginal culture 
4. physical examnation of productive system
5. ultrasound test