Sometimes my heart does not beat for a second.i will experience severe pain in that moment.it will be alright after two secs. please give me its cause?

PVC's: You are having missed heart beats. The reasons could be many. My advice is check with your doctor and get basic testing, including ekg, elecyrolytes, TSH and later may require 24 hours holter monitor.

Premature beats: What i believe you're describing are premature beats: the heart pauses after an extra beat to resume its original rhythm. During the pause (milliseconds), there is a longer than normal filling time causing the next beat to be unusually strong and may be perceived as painful. This is called "post-extra-systolic potentiation." it is not harmful or dangerous.