spotting is also a sign that the lining is "getting ready" with the fertilized egg in place. Not to be mistaken for a period. HPT can give you negative results even 2 months after a missed period if done improperly. If she's been drinking a lot of water, her urine may be too diluted to show the hormone in increased concentrations. If she's taken it at night, the hormone may not be built up enough either. Best time to take the HPT is in the morning when you first wake up, and not to drink a lot of water the evening before.
During the missed period remember to look for other signs including:
Swollen and sensitive breast
Bloating in the abdomen (Not weight gain, just tighter fitting pants from bloating)
Super smell (really sensitive nose)
Lack of energy
Food habits changing
SPOTTING
Hang in there; and best advice I'll give you is treat your g/f as if she's pregnant. Keep her away from Caffeine, alcohol, or drugs.
I'm a chick and I don't know this much about periods!