I’d start by using a spirit level to check that your saddle is still at the same angle on the trainer as it is outside – I recently developed a bit of a sore spot and apart from the saddle being knackered from tens of thousands of km, it was also slightly nose up on the trainer. I changed the saddle and levelled it out by putting some cardboard packing under the rear rocker plate and it’s now appreciably better.
As others have said, the static nature of an indoor trainer amplifies any fit issues. If you’re not doing already, make an effort to get out of the saddle for a while every ten minutes or so. I suspect the optimum answer would be to have a bike fit, particularly if it’s a road bike, but it’s quite an expensive option.