Yawning is not a sign of pregnancy…you can never depend on the "signs and symptoms" of pregnancy, the most accurate symptom is a missed period and a positive HPT but if you are coming up on your due period and are late then take a test.
def chance you could be preg, sometimes dark spotting is from implantation, also you can have very light period like bleeding at the time you were due for your period as the hormones havent built up enough, this can go on for a couple of months each time your period would be due and you can still remain pregnant, time to do a test :)