I have had panic/anxiety attacks for 12 years. They are awful. I have been on Phen for about 3 weeks and haven't had one. I do take klonopin when I have one and it "brings me down". I had heard that about B vitamins too. I took them back in the day and it did help. I have also tried Valerian Root and Melatonin, both which helped as well.
It sounds like your heart is "overreacting" to the amount of stress caused by activity like running. It really doesn't sound like a panic or anxiety attack to me. More like palpatations or something of the sort. A panic or anxiety attack will come on slowly or suddenly like Goth said and will make you feel this horrible sense of impending doom. You feel like you will die. You feel smothered and scared. It is awful! I would talk to your doc, but either way, I really hope it gets better!!