Bluesapphire, from your listed symptoms I'm gonna take a stab at saying that whichever diet pills you took (assuming they were an OTC dietary supplement) contained either one or a combination of stimulants like caffeine,ephedra or another ingredient like yohimbine. These ingredients can cause the tachycardia that you experienced and the general feeling that you are about to die...

If ever you decide try a diet pill product again, ensure that they contain no stimulants. A really effective stimulant free one that I know of is a product called Zappetite (yeah I know the name's cheesy) made by Thermolife . My sister suffers fthe same side effects from stim-based diet pills and she takes these. It cut her appetite quite a bit, gave her a bit of energy and she's lost and managed to keep off about 20lbs. It contains OEA and Hoodia. It's worth a google. Also, Alli might be worth a try but if it's anything like it's parent drug Xenical, you'll want to always be near a bathroom