I'm not quite sure how to explain this, but yes like you said THC is THC, however depending on the type of marijuana it will have a different impact on your body. You like Indicas, so that means you prefer the "relaxed, entire body is calm" high. Personally, I think that your tolerance to THC is simply too high. Take some time off smoking herb, and when you get back to it it should be much better. Or maybe you're just getting bad bud recently.