Brown discharge mid-cycle for a few days is usually just old blood making its way out slowly, often linked to ovulation spotting or a small hormonal dip around that point in your cycle. Since your cycles are otherwise regular and this is a one-off, it's not typically something to worry about.
Next Steps
Track whether it repeats next cycle and note any pattern with your ovulation dates. If it settles on its own this cycle, no treatment is usually needed.
Health Tips
See a gynaecologist if it turns heavy, smells bad, comes with pain, or continues into your next cycle, since only an in-person exam can confirm there's nothing else going on.