Looks like you may have just stretched it a bit too much. Should be a pretty easy fix though.
Try turning on the wireframe first. Then use the reduce brush a bit on those areas (and possibly a bit of the smooth brush if needed). Once the tris are a bit evened out, just use the grab brush again. This time not to move the area, but when you are zoomed in just clicking on an area with the grab brush will increase the triangle count within the brushes area without effecting the topology of the mesh. Then you can go back in and add as many details as you want on a more even surface.
I’m assuming that will fix your issue, but if not just post again and I’m sure someone should be able to figure it out.