So, first off, I stopped my curiosity in the aspect of magick a while back because of a frightening experience I had, I was trying to AP and it wouldn't work, so I tried again, only the second time I saw a being that I felt had a 'negative' energy to it, I don't know if this was a warning or what but it effected me and I stopped for a while. So, is this common at all?
Usually those who carry warnings don't feel negative they usually feel positive, or neutral, from my experience anyway. I would also suggest a protective spell before trying to project if beings bother you when you are projecting, casting a spell before hand have helped ward away negative beings for me. And for being common I don't know for sure but it has happened to one of my friends before and me.
This may not have been a negative energy, it could be just fear. Yes, it could very likely be a warning from a spirit guide. If you feel that this was a negative energy, you may want to try protection rituals or spells as Phoneix stated. I wrote a post about tips, on Astral Projection, I hope you find it useful for your experience: