Do you have a guru? Have you been given wang/lung/tri for the yidam(s) you are reciting the mantras of?
In your case it was your meditation beads that turned into the centipede which suggests you need to look at your spiritual practice.
I think Freja is onto something here. In particular, since centipedes are associated with poison within Vajrayana, I'd make sure to closely examine my practice of mantra and my state of mind during said practice. Perhaps your dream was a warning sign that your poisons are increasing, rather than decreasing, from such practice.
I must confess that, unfortunately, this was the case for me for a long time. I'd do what I felt drawn to, rather than what my gurus instructed me to do. Not only did this mean I was hopping around from practice to practice on a whim, but also I was creating the path as I went along. In the end, I read an excellent passage from Chögyam Trungpa Rinpoche in the Intro to Jamgön Kongtrul's Torch of Certainty
, and realized that this behavior was reinforcing my ego, not to mention reinventing the wheel.
"The Sutras, Tantras, and Philosophical Scriptures are great in number. However life is short, and intelligence is limited, so it's hard to cover them completely. You may know a lot, but if you don't put it into practice, it's like dying of thirst on the shore of a great lake. Likewise, it happens that a common corpse is found in the bed of a great scholar." ~ Karma Chagme དྲིན་ཆེན་རྗེ་བཙུན་བླ་མ་རཱ་ག་ཨ་སྱ་མཁྱེན་ནོ།