What Causes Delayed Periods With Pregnancy Test Negative?

Should I go see my OB/GYN if my period is over 43 days since last menses, I took an hpt that came out negative and one at my regular doctors and haven`t heard from them or been able to get through. I am 19 & 5 months postpartum, my menses have always been regular only usually off by 1-3 days. Except for when I was prego I never missed a period even under extreme stress, exercise, etc... I don`t work out anymore and my life isn`t as stressful as it once was so idk why I would miss a period besides either being prego or something else is wrong. Thank you for your time!