Sounds like it is the start of a kundalini awakening. I had something similar happen while stoned. It was more of an intense warming sensation. My best advice is to just lay off meditating while stoned for a while. If you really want to do it while stoned, then your best bet is to just limit yourself to maybe 15 min sessions. If it's not that big of a problem for you then by all means continue. Again I'm no expert, but it does sound like something spirtual is happening to you but remember to not pay too much attention to it. Let it be and move on.