Please include a picture that includes the mesh. Maybe turn on the 2D+3D view option in Paitner and grab a screen shot.
LIKELY CAUSE:You probably have a very "dense" UV map in the streched area. The vocabulary term is, "Texel Density."
For example, the 2D mesh may only cover 10 pixels, but then gets streched out to 75 pixels on the 3D model. In that case the best thing to do is to "unwrap" the model again and make a more effective UV map.
Did that make sense?
There is a tutorial that discusses this in more detail. I've linked to it before. If I find it again I'll post it here.
Ah... found the video.
The first 3 minutes will introduce the topic quite nicely without getting too deep, but you will want to watch at least until the 5-minute mark:
https://www.youtube.com/watch?v=bwusznnkRxo