Never had them, just looked it up (didn't see your thread, sorry for the late reply). Seems like you can get Basal cell carcinoma from sunlight and a weakened immune system or even a family history of cancer.
http://www.mayoclinic.com/health/basal-cell-carcinoma/DS00925/DSECTION=causes
I don't know if it was the meds but I do know that you should limit the amount of time you're in the sun with 6MP (and possibly the other meds, certain ones are known to increase your risk or cancer, but what doesn't these days?) or at least wear a 30 SPF or higher sunblock (they have a spray now which my work well on your head).
I know my grandpa gets some type of skin cancer on his head (probably that) and his GP just freezes them off. There are many different ways to treat it so I'd start with removal and then increase prevention (read through that link above) and tell your GI about it if you haven't already and if it keeps happening then who knows, you may need a med change but I'm not certain.