Why does smoking make so sleepy?!

I don't usually smoke. But my friend gave me a pack of Malbros, they were different though, with Spanish packaging. Anyway I smoked one, and could barely walk after, I was all limp and floaty and tiered. Is it from lack of oxygen? Or were they tainted? I smoked another one while out for a walk, and was sitting on a tree branch, and when I hopped off, I couldn't walk, I had to lean against the tree for a few minutes. And if it makes a difference, I inhale/smoke them prpoerly, take some, then take a breath of air, then exhale. Then I fell asleep. I find this very stranged. I've smoked before, though only a few times and had been drinking so I didn't notice any difference. Sorry if this is a wrong section, they don't have a section for smoking.