How long can a girl's menopause delayed?

We did it on the 26th day from the first day her period started, and we had used a condom. But this month her period is delayed by almost 8 days. Today is the 38th day but the period hasn't started yet. Few days earlier though she had a blood spot which didn't continue. This has made us more worried. We carried out the home pregnancy test, the result is negative though. Can anyone suggest, if she might be pregnant?