What Causes A Delayed Period And How To Get Regular Periods?

I have a baby of 8 months old, he is our first baby, since 2 months my periods have started after first delivery, they were regular march12th and april 13th are the period dates., but this month period hasn't come and i have done pregnancy test for twice one week before and again today the result is negative, then what is reason for the delay in my period.. what can i do for occurring regular periods.. how can we control pregnancy other than using condoms.. or only using condom is the best method?? does this method gets fail?