If you take aspirin as a blood thinner and then take it for pain do you overdose?

The amount of aspirin usually "about half an aspirin", taken to serve as a blood thinning agent is very small compared to that used for the treatment of pain, so the difference it would make should be negligible, However you can allow for it by taking the amount taken for the blood thinning from the dose for the pain. In any case you don't have anything to worry about.