Your negative experience was brought on by the drugs and your anxiety made the experience much worse. You may have ended up having a panic attack, but it sounds like you started off getting the chills brought on by drugs and you freaked out because of it. You probably should avoid dexedrine. I've sworn off amphetamines for good and my life is better because of it.
Messing with dopamine levels (which happens directly when taking an amphetamine and indirectly when taking an opiate) can cause your body temperature to either raise or drop. After dosing on any opiate I normally wake up in the middle of the night sweating like crazy. I've also had many of these episodes of the chills, but normally I don't freak out. I try to relax my body and I don't feel like I'm going to die or anything, I just wrap a blanket around myself and warm up and then I'm fine. Your warm bath worked.