How do you know your entering perimenopause for sure? For the last 6 months my peroids have been coming earlier than expected, usually 3/4 days early and they have also been shorter in length. Last year my peroid would last 6 days. The last 6 months its been 4/5 long. This month has been very odd. It came 6 days early and only lasted 3 days. Im not sexually active at the time ( havent been for a few months) I'm 37, and started my peroid at 9... could i have entered a new phase?