Anything sitting in the water (like a teabag) has stuff in it that will fester and rot and grow bacteria when sitting in water at room temperature. If it was a couple of days, I wouldn't worry about it, but a month? That's really gross, I would definitely not drink that. Doesn't matter if you cover it or not, there is already air in there. And even if you didn't leave the teabag in, it WAS in there. Even a tiny bit of something that contained bacteria or microbes left in the water will cause it to multiply is given air and water. Unless you pumped all of the air out with a vacuum and replaced it with inert gas before you put the lid on, there is air in there.
If you got sick the first time, it was probably because opiates can cause pretty bad nausea when you take too much, and when you take too much it's also not really euphoric, either... it mostly feels unpleasant. If you have drank it recently and gotten sick, it might literally also be getting you sick, from nasty stuff that grew in it.