I have a 30-day cycle. ovulation days 12-16. on day 17 of cycle (today)had tiny amount of brown mucus spotting and 1 smear of red blood. why is this?

When in doubt: just check pregnancy test in 2 weeks and repeat if negative in 1 week. And if you are still not bleeding normally and first pregnancy test was negative, check it again in 1 week. Try to remember if the last time you had unprotected intercourse was also near your last ovulation date. Then go see your GYN or family physician to get examined for infection if things persist.