Late w/ BFN

Tiami • Married for 6 yrs & ready for a family!
I am 5 days late and AF is usually regular. This morning I took a cheap ept (wal-mart brand) and I KNOW I seen a positive but when I checked 5 mins later it said negative. Or the lines are so faint I just can't see. (I didn't have my contacts in either) Has this happened to anyone before?