Yesterday, my legs felt like jello, i felt slightly dizzy and nauseous with dull left arm pain and migraine. i am on my period. is this a heart attack?

Probably not: This would be rare in an 18 year old, but you should see your primary care doctor for an evaluation.

Not a heart attack: Gosh. You are only 18 and you thought you were having a heart attack? There can be a thousand different reasons for your experience. On the period you could be losing more than normal amounts of blood; you could be not eating enough; it could be hot where you are and you are a bit dehydrated. The last thing on a list of a thousand possibilities would be a heart attack.