I'm 45 years old female, i'm on birth control and rest 7 days during the 7 days i'm on my period after i start my pill again i start to spot. why?

Two Reasons: Unfortunately, you are in your mid-40s which are notoriously a bad time for periods: it's like the storm before the calm sometimes. You'd think things would wind down a bit, but periods in these years may be more irregular with more trouble regarding duration, frequency, & amount. All of this should be controlled, however, with the right pill. Tell your fp you need a pill with more estrogen in it.