Why am i vomiting a little after snorting ritalin?

I have doen this for years w out any problems, but recently stopped as much and started doing one pill not two. And after 2 weeks of not doing it, I snorted a pill..and 20 mins later vomited a littttle. And it happened again later that day when I did another..this has never happened before, and my friend thinks my body could be rejecting it as a sign of early pregnancy, but I doubt itt. Iffff annyone knows anout this or had it happen pleaseee help me realize why all of sudden i vomit a little after snorting a pill?