Since you are 45 years old and pregnancy
is negative,the most likely explanation could be the start of peri menopausal period in you.
This is a period when body gradually changes to menopause
and there is gradual reduction in the production of female hormones like estrogen and progesterone
This transitional period from peri menopause to menopause doesn't happen overnight and may take many years before finally periods stop.
Missing or irregular periods
may be the first sign of this premenopausal period.
Other reasons for delayed periods are stress,depression
,severe illness or excessive loss or gain of weight.
However you must consult a Gynecologist for proper evaluation and diagnosis of cause.