It is completely normal. You added a bunch of sugar (in the strawberries) so the yeast is fermenting it.
Also personally I do not recommend pureed strawberries, although if you are fermenting in a carboy you may not have a choice. In my experience, it is nearly impossible to avoid getting little bits of strawberry in your beer with pureed strawberries.
I recommend using a bucket for secondary, washing, halving and freezing the strawberries in a sanitized container, and then putting them in a paint strainer or hops bag and racking onto them in the bucket. It is all around a lot easier to deal with them in a self-contained bag like that.