All three answers on this page take the stance that masturbation cannot be an addiction. Since this is a controversial topic, I'll play devils advocate:
Neurobiologically addiction is marked by
- Stress surfeit
- Reward deficit
- Stimulus-response perseveration
- Self-regulation disorder
(I got this from Dr. George Koob.)
So if masturbation starts losing its appeal, and is just done because it's stressful not to do it, then yes, I'd say that's evidence for the underlying neurological changes of addiction.
As for the comment that an "urge for a behavior does not make that behavior an addiction," I agree but I think urges are tightly coupled with addiction formation. An urge is evidence of a mesocorticolimbic association between a stimuli and response. When this connection is recognized by society as an "addiction" is just a matter of degree.
Perhaps, if you could measure your subjective experience of masturbation over time, you'd have a better idea of when you became addicted: specifically when masturbation only gets you to your former baseline levels of satisfaction. So yes, maybe the urge isn't enough to call it addiction. But it's at least evidence that your brain is busy forming one.