Panic attacks, and similar Anxiety attacks are more common than most people think - I think I remember reading that around a third of the population suffer from them from time to time. They don't do you any harm but they are incredibly uncomfortable to experience. Fear of dying or having a heart attack are common symptoms believe it or not. It's more scary not knowing what it is that's happening but finding out more about what's happening helps. And also remember to think about your breathing - you may feel that you aren't breathing fast enough when you are in fact hyperventilating which, if it carries on long enough could make you pass out. It's easier said than done but try to reassure yourself during an attack - tell yourself it's only a panic attack, nothing bad is going to happen and try to relax. There are some herbal remedies which are very effective for some people - try asking in Holland and Barrett or somewhere, and since it's herbal it won't do you any harm. Bach Rescue Remedy is one worth trying.
Anyway I hope you're OK