Your heart rate is less than 100 beats per minute, so this is not a tachycardia (by definition). As you indicate you are also upset when you have this heartbeat the most likely explanation is some sort of panic attack. Instead of taking Lorazepam, which can be addictive, you may want to consider taking a low dose of a betablocker (talk to your doctor about this) just before going to sleep. Unless you have a very low heart normally or for very specific heart conditions, these are generally better tolerated long term than 'sleeping pills' (like Lorazepam). You might also want to look up information about 'panic attacks' to see whether you think your symptoms match, and talk to your doctor if you think they do.
hi i am a 21 years female and i have a similar problem the only difference is that i don't wake up through out the night i will lay down to go to sleep and right before i go to sleep i will have to get up and walk around because my heart rate speeds up and i have two things one i am aerophobic which is the fear of not being able to get enough air or aka not being able to breath and then i have panic attacks and when i get shortness of breath then that triggers the aerophobia and then that triggers the panic attacks so i know what you are going through when doctors tell you that you are fine and healthy but you feel that there is something else wrong the only thing that i have come to find that helps me besides the meds is that if i go into a room by myself and sit there for a few mins and just breath take long deep breaths and that seems to get my heart rate back down and every thing is OK again well i hope that this helps if not i would see about going to another doctor to get a second opinion