This is strange. Sounds like you got some grout with latex in it.
I always use premixed grout that is sanded or unsanded.
I mist the new grout three times a day as directed for three days. This is part of the curing process.
Then I tell customers not to silicon the grout for 10 days. And also not to use the shower or area for three days.
There is a vey novel test for the silicon job. Do you know about it?
you apparently have a defective grout and you should stop all showers and have this removed and replaced with good stuff. If not, you are going to have major problems with your flooring or what ever is underneath the tile.