Acid flashbacks or flashbacks from other serotogenic psychedelics are essentially a myth, and it's not stored in your spine....... and you simply cannot begin full on tripping again no matter how many times you crack your back. I promise. No psychoactive that I am aware of could be stored in the body for long periods of time and then released again. I can't definitively say that there's no way that such a drug exists, because there are new substances being created all the time. However, it would be very rare if it happens at all...... because as others have said, the body is very good at getting rid of psychoactive substances.
As far as the mind changing state, it is most likely all in your head. If it is causing you problems or you are concerned about it, I'd recommend seeing either a doctor or psychologist/psychiatrist since naturopathy is primarily a field of con artists IMO. However, it probably isn't related to substances being re-released into the body. That would be very rare and may not be physically possible even theoretically.
As for the MDMA "flashback", I've experienced similar things after tripping. Not with MDMA as I haven't done it
yet (definitely would though if I came across it
), but I have had psychedelic like states while high. It's just that somehow you're able to flip into certain aspects of that mindset and make it seem like you're tripping. After I've tripped, I've even had mild visuals while high. Not full on psychedelic visuals but just CEVs,
really sharp vision (e.g. way sharper than weed would make it prior to tripping, maybe even better than 20/20 it seemed), and even psychedelic esque colors/slight moving and shifting of objects. Many friends of mine have also said that weed changed for them after tripping, and I have heard similar things from some people I knew who took MDMA. That it's different after psychedelics. A friend of mine who used to do a lot of PCP said that he couldn't even smoke weed anymore after he stopped taking PCP because he'd feel like he was back on PCP again and start freaking out.