Brown or dark spotting like this is usually just old blood taking longer to leave your body. It's a common pattern before a period, especially with your longer 35 to 45 day cycles.
Next Steps
Watch whether this turns into your regular period over the next few days. If it doesn't, or this repeats most cycles, ask a gynaecologist to check your hormone levels.
Health Tips
See a doctor sooner if you notice pain, fever, or an unusual smell, since those point to something other than a normal cycle.