It usually happens if you have not been riding for a long time, after training for 2 or 3 weeks you get used to it and it no longer hurts

It depends. Incidentally, I’ve noticed that it takes about 20 minutes or so for the comfy soft saddles to become uncomfortable- when your sitbones sink into the cushion, it presses up into soft tissue and that takes time to start hurting. By contrast, if you’re sitting bone-on-firm-saddle, that takes seconds to become uncomfortable.

Perhaps counterintuitively, the way to be comfortable for long stints in the saddle seems to be to get used to sitting bone-on-firm-saddle such that blood flow into soft tissue is unimpeded. It can take a few sessions to get used to it (and it requires a saddle matching your backside, fit to you on the bike).

It can be hard talking about it because it involves personal anatomical bits but probably your best bet is asking if your local bike shop can recommend someone to help you with your bike fit.

Where on your butt? The pressure I feel on my bike seat is on the same place as when I’m sitting on a hard chair. You might want to cinsider adjusting the position of your seat in this way.

I know this post is old, but I wanted to comment on it for anyone having this question. The issue op is experiencing is most likely do to the seat. Op should replace seat since it hurts after only 20 mins. However, the information shared by RAF is something I experienced before. It should be same region as when sitting on hard and flat chair. If that’s not the case, then maybe see doctor.

In my experience this normally means the seat isn’t adjusted properly. Get a level and set it to 0° and give it another ride. If it still feels bad, make sure you’re sitting far enough back that your arms are stretched out, but not so far that your elbows are locked and that you’re sitting on the correct part of the seat. If you walk in most bike shops I suspect they’ll fit you for free too.