Hi TNTN,
No, it is not a sign of imbalance, it is a sign that the energy body (pranamayakosha) is activating and purifying. There is nothing wrong, even if it feels like a lot is happening and there are a lot of sensations. If you started to feel pain, or discomfort, that would be a sign of over-purification, but that does not seem to be the case here.
Whereas savasana can activate more movement of prana, sitting is better for going inwards into silence. This is because if we are lying down, it is easier to simply fall asleep, whereas when we are sitting, the prana supports the body and we can go deep into stillness whilst remaining awake and alert. So sitting is the key to enlightenment, whereas lying down, tends not to be. For that reason it is done after sitting rather than instead of.
And yes, it is quite possible to bathe in massive doses of blissful prana, as you put it, without experiencing any negative symptoms. It is one of the aspects of a largely purified subtle body.
Christi