Why is my cycle long?

My cycle is between 30-38 days but why is it different every month? look i started m period:
January 30th-Feb.8th
March 8th-13th
April 15th-20th
May 17-ill probably end the 21st. (my period is always 5 days long)

Why isnt my cycle the same length all the time? i dont know when i ovualte because of this.. when do u think i'll ovualte this month? please help thanks.
No i started my period 4 years ago
 
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