this happens to me sometimes as well, a histamine reaction is likely. with an opioid histamine reaction though, there should be some other affects.
when this does happen to me, i figure its just my body rejecting the substance, telling me that it doesnt agree with the chemichal, or that there is more then it can handle, sensing danger and trying to get it out the only ways it can; a minor literal overdose, or volatile intolerance to a 'perceived pathogen'; vomiting, diarrhea, profusely sweating, salivating excessively, chills etc.
if you experience delerium, hysteria, confusion, and other physical sideeffects, this could be an anaphylactic reaction. some have predispositions to developing such an allergic reaction to a food/(bio) chemical. i doubt this is the case for you, but is nice to be aware.
cotton fever gets pretty awful too, is likely, but ongoing and more violent. from what ive heard anyway.
be safe.