Yes that's the heart chakra, it feels like it's pulsating incredibly fast. If this shocks you it will remove you from trance so you should try and relax into it.
My brother has gone through this too, only that he said that an evil entity in their room, my other brother who also meditates can feel and hear this entity, so it scares them a little.
Yeah happens all the time, completely normal to be freaked out by it also as it when it happened to me made me think I was going to have a heart attack.
It also suggests your pushing too hard, astral projection is an advanced skill and trying to force it can be very bad.