What Causes Irregular Periods At 40 Years Age?

Hello, I am very worried with regards to my periods. I have had two late periods in a row then my last period was on time which finished 10 days ago. I started to bleed 3 days ago with cramps, it is not as heavy as my normal periods and i have noticed it is brown in colour then bright red and pinkish brown. I visited my gp 4 weeks ago regarding this and she sent me for a blood test to check for perimenopause which has come back fine, i also had a fbc test a year ago which was normal. I am just concerned why my periods have started to play up and what is going on with them. regards julie bond.Aged 40.