If you haven't taken, eaten or drunk anything unusual, perhaps you're having a panic attack?
Panic attacks are often triggered by anxiety, and they can make you feel like you're about to have a heart attack. You may feel short of breath, shaky, nauseous, sweaty and have palpitations. They can feel very scary; a panic attack sounds like a simple case of 'the nerves' but it can result in very real physical symptoms.
They can be hard to recognise whilst they're happening and they can take quite a while to calm down - the best part of a day in my case - and I've found the best thing I can do is to just head straight home to lie in bed in a darkened room for a few hours where I usually sleep it off.
If that doesn't sound like what you're experiencing, it'd be a good idea to get a doctor to check you out.