Those 'diet pills' I assume you mean the over the counter ones you can get at gas stations and probably even some head shops are essentially legal and/or herbal stimulants. Basically fake speed I looked at some diet pills compared to some energy pills the other day and they pretty much all had the same ingredients. Stimulants can cause anxiety symptoms for sure, especially if you already have anxiety issues or overdo it like take 3 when the package says to take one. I wouldn't say it was stupid of you just a little mis-informed. I sometimes take such things for energy but I am always prepared for the potential of it adding to my anxiety.

are you sensitive to caffeine? anyways it should wear off on its own just make sure you drink enough water if you have any chamomille tea or any relaxing supplements those could help to calm you or if you have anxiety meds but that might not be the safest choice depending on what was in the diet pill. Also though if uhh you haven't recovered yet and feel you are in danger obviously call 911 but other then that it does wear off, sometimes the come down can be unpleasant though.