You may have accidentally selected a different brush. This can be changed under the brush presets panel. Hope this helps.
The brushes you are using also may be or a low resolution. E.G. They were created in a small size, and making the size bigger than they were created to do will change the clarity. Just like low res files do when they are added onto a high res file.
I had this problem and found that I had accidentally changed mode from "normal" to "dissolve". When your brush tool is active, check that the mode box on the top menu shows "normal". This solved it for me.
For me I just went to reset all tools and it fixed the problem.
Image > Mode > choose higher B/Channel
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.