Unfortunately, we would never really be able to give you a solid answer to this question without testing the drugs.
@KTFden highlighted that point and he's definitely right.
I will say that I've seen numerous examples of people using Methamphetamine chronically, develop a serious tolerance and then start talking about how they suspect their drugs are not right. People talk a lot about contamination with isoproplybenzylamine. It is true that this chemical has been found as a cutting agent in Methamphetamine before, but not to the extent that the market is overrun with the stuff. My knowledge is that Methamphetamine is cheap enough to produce on its own and that the use of a cutting agent like this has no real financial benefit in the long run. I believe it likely happened by coincidence, that someone was out of this or out of that and it made sense at that time, but I don't believe it makes sense in the long run.
I just want to say it's very common, what you're describing. I truly believe that it's usually a combination of tolerance combined with the paranoia that comes with Amphetamine usage leading to this conclusion. It's possible that you're drugs are cut, but my layperson's opinion is that it's significantly more likely that they are not cut.