It happened while i was meditating for 1-2 hours a day sitting up and then lying down on my couch and practicing for another hour or two. At some point, I looked at my feet at the end of the couch and I didn't recognize them as 'mine'. I felt in to my body and no longer identified with the sensations. Not particularly unpleasant on its own, but it was coupled with a sensation of doom that was similar to coming up on a hallucinogen and hearing news that someone you knew died. The fear and sensation of pressure and stress lasted for about 2-3 weeks, all day, every day. It was an inescapable sense of doom. I did not feel any sense of compassion or joy that I had heard arise in conjunction with my other symptoms. Eventually, the feeling passed. Now I make sure to explicitly cultivate a sense of self-compassion and my 'lows' have been a lot less fearful. I do have a history of depression and have been diagnosed with ptsd so if you're relatively healthy and generally feel okay about life I wouldn't be too worried about such an experience in everyday practice. I also remembering that maybe the event was something meaningful and 'dove in'. I kept meditating in isolation. In hindsight I could have pulled back a little bit. While I have had one or two episodes similar to that since, I understand they're momentary and instead of meditating, I distract myself with tv or something similar.
For the first year of my mindfulness practice, I cheerily recommended meditation to acquaintances who were struggling with anxiety and a lack of joy. After a harrowing experience of dissolution that lasted for several weeks, I realized that mindfulness isn't just calm and pleasant experiences. I still continue to practice and it does bring me joy but I no longer recommend this particular practice to anyone who has a rocky past and a limited support system and just wants to feel 'good'. For that, I think diet, exercise, and sleep are safe bets.