If you had two negative pregnancy tests, it is unlikely you had a miscarriage. When your period is late, it is usually due to your not having ovulated that particular month-which can happen from stress, recent weight loss or gain, or most commonly, just by chance. While you are not menstruating, the hormones in your body have a longer time to build up tissue in your uterus, so when you finally do menstruate, you have extra blood and tissue that will be expelled. If you bleed enough that you need to change a pad or tampon from being soaked more than every hour, or if you get dizzy or clots last more than a few days, you should see an OBGYN for evaluation. Otherwise, your cycle will likely come back normally once this period is over. I hope this was helpful and answered your question. Good luck with everything.