Very faint line on HPT?

I'm about 20 DPO and just had an extremely faint line on a HPT. Why would it be this faint this many days out? This will be baby #3 and I've definitely "felt" pregnant, so I thought I would get a darker line. I'm scared to get excited because I'm feeling doubtful. Anyone know what causes a faint line this many days out? LMP was May 9th.