This is probably what is termed, "Concentration Phenomena." It is very common when the mind gets very concentrated, for various different strange sensations or experiences to arise in meditation. Sometimes it might be called "Piti," or rapture. Try not to get too curious about it, because if you start to just focus on this phenomena, it can lead you astray. As well, sometimes if you focus too much on it, it may start out very pleasant but it can then become quite unpleasant.
Another example is someone who gets visions in meditation. Because it is so different, they become more curious and attached to it, they start to want to see it, the visions may be quite pleasant in the beginning, but then they become more and more bizarre and someone can even go mad if they get too involved in it. So with Concentration Phenomena we try to just see it as a natural cause and effect that the concentration is increasing. And that this is just a result, just a manifestation of the mind. Try not to become too attached and try to just note it and return to your primary object.
If you tend to get these more when you are observing the body sensations, then I suggest that you change you're awareness more to Compassion and Lovingkindness meditation at that point, it may be helpful. One teacher even recommended someone who got it a lot, just to walk, and wouldn't let them sit, just kept them walking and walking doing walking meditation just to get them out of this attachment that they had to this particular phenomena. Because every time they sat, they couldn't be equanimous to it, they'd get lost in it. So that teacher just wouldn't let them sit, until they'd walked it out. And then told the meditator to be very careful about just noting it, and reflecting on impermanence, Anatta and Dukkha of the sensation.