Do you ever do things you just decided not to do?

So I've got this problem that's confusing me a lot. I'm a chronic pain patient who takes opiates legally by prescription. Before I became disabled, I used opiates occasionally for spiritual and creative purposes. I was never an addict and never had a problem controlling my intake or behavior. I've had a physical dependency before, but always previously tapered off without the slightest problem.

Nowadays, I can't taper off entirely because of pain. But I need to keep my intake to a certain limit so I don't exceed my prescription. Yet I keep taking more than I want or need to. It's not like I'm taking enough to get high, or even enough to get more pain relief. It's just enough to be more than I can afford, thus ensuring that I'll run out before I can fill my next prescription.

Why do I do this? It always goes the same way. I sit down, take out my medicine, and decide to just take my normal prescribed dose. Then, immediately, as if on autopilot and making their own decisions, my hands proceed to take out more and make me consume more. Even sometimes I try to stop and I just... can't. It feels similar to some of my OCD compulsions, but at the same time, it's different. It's driving me nuts. Does anyone else do things like that or am I just completely broken?