For me, this happens when it seems like alot of stuff is dying.(like I'll have a large amount in stool the next day.) OR, it also happens when there has been a very long stretch in between meals.(say, 6 hrs or more) and then I have a meal. For the first few minutes my heart will beat very hard and fast. At first I thought this was liver(which it could be), but I think it's my blood Sugar rising.