Possible causes listed but not in order of priority:
1.
Saddle is not level: I use an app called bubble indicator which is available on andriod and maybe on ios as well, use it to check if you saddle is completely level. Alternatively use a horizontal line reference available, say a wooden barrier for example.
2.
You setback is too much: You need to reduce your setback (move saddle forward) such that your arms do bear some weight but "not too much weight" (you can refer here for more details on this topic:
https://www.stevehoggbikefitting.com...or-road-bikes/) . This would allow to distribute some of your weight from the bum to your hands. I prefer distribution of 60/40 which is ratio of rear weight to front weight on the frame of the bike.
3.
Saddle issues: I changed my saddle from stock to "Madison Prime" which is very comfortable road saddle. This helped me cure my bum pain. But this is one of the last things I would, personally, consider.
4. You
saddle height is not optimal. Your saddle shouldn't bear the entire weight. Reduce your saddle height in small increments. I would suggest using small segment of road to test and adjust your saddle height.
5. You should stand on your pedals sometimes if not frequently to change muscle set and allow some blood into your resting region. Possibly you could be
sitting too much!
6. Lastly, it could be you're
not used to the saddle. The saddle sores initial but slowly the body gets adapted to the position. So you may need to allow
some recovery so you can be stronger in your saddle.
7. Using
anti chafing cream when you go for a ride. It could be the
friction between clothes and your skin.
I hope all of this helps because this is from experience.
Happy riding mate.