hi my husband and i have a 17mth old and a few months ago we had an early miscarriage at 5 weeks

i did not have to have a d and c and hcg levels returned to normal. i have had 2 normal periods since then. But this month i am 8 days late and negative pregnancy tests.
Is this normal and do you sometimes skip a period and if so how to i know when my next cycle starts
i am very confused.