Lighter period

Over the last 2 cycles my period has become lighter. I'm not on any kind of birth control and I'm NOT pregnant, I haven't had sex in about 6 months and took a test in late April/ early May and it was very strong negative result. My period in May came a day early (May 29th to June 2nd) and this month I was about 5 days late and this period was lighter and was very dark brown and dark red. Any ideas as to why my period is becoming lighter? Hormones? New cycle?