Brush selected but no interaction with image- help?

After maybe 15-20 minutes of using photoshop (latest version) my cursor changes to a pointer even though the brush tool is still selected. I've tried pressing keys on the keyboard to check nothing is stuck but the only solution I have found is to restart photoshop, which is a pain to do so frequently! If I click away from the image, such as on the home icon, the whole image area freezes even though the workspace tools are still selectable. I'd love some help with this as it is a complete pain!