anything could of caused ur headache. have you consumed any other drugs around the same time? including alcohol? If yes, your dopamine, serotonin levels in your brain (the stuff that makes you happy etc) may have been depleted or not responding properly. how were you feeling on the day? was it hot and/or were you tired, this may be a reason as stated above. Just say it was hot and you had been out in the sun all day then most probably your body was dehydrated and when you consumed the methamp it would have just accelerated your dehydration, if you were dehydrated the meth could have quickened/hightened your body's awareness to it causing ur headache. in truth it could have been a number of things, noone can really tell you with 100% accuracy. it could of even been as simpleas ur body/brain not used to it. but the things i can absolutely guarentee to you about the speed/meth you consumed and any that you may consume in the future is;
-the speed/meth you buy will always be cut no matter how much the dealer says otherwise or by how much its sold or called. but it is most unlikely the cuts used would be anymore harmfull to you then the methamphetamine itself, it doesnt make sense in a money-makeing point of view to use something that makes people sick and not come back.
-all speed is illegally made, so there's no controls on how its made and in what environment. therefore all speed/methamphetamine is what you called 'dirty' or has impurities.
hope this helps you abit....