Like any medication, you can use too much on occassion. The THC may have been more then you are used to, or it may have been laced with something. Alot of commercial grade cannabis is being laced with JWH and it turns your high into a low fairly quick. I have also heard of Acitone being used as a drying agent and will also cause weird reactions. Its always important to know what you are smoking and not just buy it from the guy down the street. You never know what they are doing. If you ever have "chemical" type of taist after smoking then it is most likely laced with something. You may want to back off it if that starts to happen on a reguler basis. Hope this helps.