breastfeeding, periods, and negative hpts

i have breastfed all my babies for the entire first year. with my first two, when baby was 6 months old, my period came back straight to normal, every 28 days, lasting 7 days (my normal). then when baby was 9 months I got pregnant again and got a positive test on the day of my missed period. easy peasy.

now, with baby number three, still breastfeeding and she's sleeping 12 hour nights with no feedings, my period came back when she was 7 months old. it's been 4 weeks so I assumed my next period should be here. I've had cramping like I've been having a period, fatigue, breakouts on my face, weird dreams, but now I'm 3 days late (which has only ever happened when I've been pregnant) and I got a negative on my hpt! should I be concerned that I'm pregnant? do I need to go to the dr? I know it hasn't been the norm for me, but is this still in the realm of normal?