If you use a given recreational drug (or class of drug) regularly for an extended period of time, then in most cases your body eventually gets used to it and adjusts the appropriate receptors to bring you back to baseline; thus, someone who's been doing Adderall every day for several months will typically require much more than someone just starting out to get the same effect. Similarly, if you take a break from a given class of drugs for an extended period of time, then your body will eventually get used to operating without it and the amount you'll need to get high will dramatically decrease. Starting up again after a break is often a fairly dangerous time for people who aren't careful, as it will most likely take much less to get you high than you're used to taking. In your case, I don't know how much mephedrone you took, or how long you stayed off of it before going back, so I'm going to throw some numbers out.
Erowid claims that a common dose of mephedrone is somewhere in the neighborhood of 1-200mg, and a lot of the experiences list doses between about 100 and about 250mg. If you've given it a few months, then it would be wise to start in that range first to benchmark yourself; now, if you went and snorted half a gram and/or decided to re-up every hour for the entire night then you probably went too far and gave yourself a bad time. I'd be surprised if you did permanent damage (given that even stimulant psychosis tends to resolve reasonably quickly with a few benzos and decent psychiatric care), but you may have done some cardiac damage and you probably had a rough time. For future reference, if you're going to go back on a substance after an extended break, then start with the first-timer dose and then adjust from there.
