layering Issue with brush strokes

After letting someone use my photoshop the behaviour changed and I don't know how to change it back. So I make 1 brush stroke in 1 colour, that's fine, however when I make a second intersecting brushstroke on the same layer in a different colour; it goes underneath the previous one as though I'm painting on a lower layer, not on top as it did before. I'm assuming this is due to a setting I can't find, plesase help. The only way I can use photoshop now is by creating a new layer every time I change colour and merging them after (as you can imagine this is infuriatingly tedious to do).