why is my cycle getting shorter and shorter????

[thelynnfamily1999]

OK...so for YEARS I have had the standard 28 day cycle, on the dot....day 28 every month! But now I have noticed for the last few months I have 21-24 day cycles. I am so worried that my LP phase is getting so short that by the time I lose weight and start ttc #3, I will not be able to. What do you think it is? Do you think it is age ( I am 31, almost 32) or weight (I am 220 pounds)....thanks