That happens sometimes man. Not too often, but when I smoke too much sativa, it will do that to me. It's not any fun, so I've watched what I do a little more closely. My advice is to stick with indicas (they're my favorite anyway), and when you do have a sativa strand, start slowly with just a hit or two, and work your way up. Some sativas can creep up on you, making it easier to smoke 'too much' (of course you can't overdose, but for some reason it just doesn't agree with me in large amounts).