It could be anything, but you sound as if you would know the effect
between pure MDMA or some drug cocktail.  The first place to look
would be right out in the open.  Although there have been no valid
studies showing *safe* recreational use of MDMA does damage, who knows
for sure?  You mention the two specific conditions of headaches and
stomach problems.  The seritonin levels and system in the brain are
thought to affect digestion and migraine headaches for example.  Hell,
lsd was created in the search for a better headache medicine.

For the most part, people selling MDMA would not benefit from
poisoning their clients.  There could always be exceptions though.
Most rumors concerning drugs are baseless.  As someone stated earlier,
it would be hard to poison someone using LSD, and I doubt it is
strycnine(sp?).  Assuming it is a clean product, and MDMA toxicity
isn't the issue, the next viable situation would be that it isn't
MDMA, but one of its chemical brothers which could be much more toxic.

Be safe and use common sense if the symtoms get worse or continue.