After masterbating how many times in close interval? If only once, there shouldn't be any pain, but if you've done it several times in close proximity, you will most likely experience some pain, but it shouldn't be severe. This is from personal experience, not based on any scientific knowledge.
As dino said but sometimes if you do the start-stop-start-stop long session you can get a bit of blue ball or if you go at it really hard. some times if you clamp your thighs together during orgasm that will cause some pain but thats usually more around your taint area.