I am now 19 days late for my period about a week before i was due for my period i had a little light brown discharge only when i wiped.and it only lasted for a day or two which i thought may have been implantation. now its been 19 days almost 3 weeks. i took a preg. test when i was 2weeks late and it was negative. now my breats are starting to get real sore and real bad lower back pain.i dont want to make a big deal of it all so how long should i wait before i really go and have a blood test done to tell for sure??