What you took obviously was not MDMA. MDMA takes approximately 20 minutes to take effect. If it took longer, what you got was an inferior and more dangerous product - likely PMMA. Its more dangerous because it takes longer to kick in and the high is not as euphoric or psychedelic. It may give you a happy feeling, but it will also make you feel uncomfortable in your own skin. Physically and mentally unsettled - if not outright distressed. People will take more of it thinking they didn't take enough or they've built up a tolerance and need more to get the feeling of true MDMA. By the time the PMMA kicks in, you may have taken to much at this point.

Calm down though. You'll be ok. Unless you're a frequent or chronic user, its unlikely you've caused permanent damage. You may have killed a couple brain cells like with any drug or alcohol, but nothing really significant. Nothing that will give you a mental disorder.

As for focusing on your heartbeat when you smoke and why your pulse may slow or quicken ... well thats just what smoking does (cigarettes or pot). Smoking increases blood pressure and heart rate. Its that simple. If you smoked a lot everyday, your body would have a higher tolerance to the stimulants and you would notice these effects less or not all.

Depending on the strain, pot can either increase or decrease anxiety (most kinds increase it btw). Also depends on your bodies tolerance. If you used to smoke everyday, your body's tolerance level probably compensated for this and you didn't become anxious or paranoid. If you just try it once in awhile, well there's where you're going to suffer from anxiety (especially if you're already prone to anxiety).

Also, at 18, since your brain is still developing doing drugs will affect you more. If you are or were a chronic user while your brain is developing its going to have much more long term adverse effects on you than if you became a chronic user as a fully developed adult. However, developing brains can also repair and heal better.

If you feel like your brain is cluttered and your not remembering stuff as well as you used to... well the more you engage it, the less you will feel like that. Partying, doing drugs and hanging out with friends like most 18 yr olds do, does not engage your mind. Depression or other mental illnesses also clutter up the mind. Try memory or speed games or sites like Luminosity. Seriously, it works.

Also, if your really worried about it, I think its important you go to a doctor and get checked out. You could have underlying health problems that may also be affecting things like blood pressure or vision.

Drugs are bad, mmmmkay? Don't do them boys n girls!