7 days late. Negative tests.

I have 2 kids, aged 3 years old & 7 months old. I’ve had regular periods my whole life, every 31 days for 7 days. I tested 1 day late, 3 days late and 7 days late and all are negative. When should I make a doctors appt to see why I haven’t gotten my period? Should I wait like a month (if it doesn’t come at all)? I’m not ttc either but if it happens , it Happens is more how we are with both kids. My husband and I had sex once 3 weeks ago after ovulation.