Problem with opacity and brush size??

Hi everyone, I Downloaded Krita about a week ago and had been learning to use it. Everything was going well. Then when I went to continue a piece of art yesterday, despite opacity being on full, the paint is all coming up transparent on the picture.
Also the brush size is not increasing as large as it should when adjusted. I’ve tried using different brushes/ink/pixels etc. but doesn’t make a difference. I don’t know if I’ve pressed something that has caused this? The colour is just coming up weak and diluted as if I’ve turned the opacity down but the opacity is on full?
It was not doing this the first few days of use so I have no idea why it’s suddenly started happening. I’m really disappointed as I’d really enjoyed using krita , but this issue is preventing me being able to carry on with my artwork.
Any advice would be great, Thank you!

Hi

The best would be to provide us a few printscreens which shows the brush you are using and the layers dock.

If things become transparent while painting it could mean that you are on a transparency mask or you are in ereaser mode.

A screenshot of the brush editor and of the layer docker would be helpful. Which brush preset are you using. Does it affect all the brushes?