heart will never stop for 15 to 20 min as person cannot survive that long time if heart completely stop working .
i feel that heart rate
may go lower side and you may feel that it stopped,
if your routine EKG is normal then i advice to undergo 24 hour holter study test ,
this test will record your heart beat for 24 hours and it will give you report of minimum heart rate as well as maximum heart rate in 24 hours also
if your holter report also comes normal and still you have problem then you consult electrophysiologist .