Why do I keep spotting on and off?

Depends. If you are spotting mid cycle between periods, it is likely due to ovulation. If you are spotting off and on and not having periods, then it is likely anovulatory breakthrough bleeding - and you should see your doctor for treatment options. If you are spotting off and on and having regular periods, you need to see your doctor as you may have a polyp, infection, fibroids, or endometriosis.
Pregnant or not? In a nutshell, there are 2 types of spotting (abnormal bleeding). Either from abnormal hormone production or from a lesion in the reproductive tract (tumor, polyp, infection, abnormal pregnancy, etc.). This answer is greatly oversimplified as causes depend upon age, pregnancy status, physical exam, etc. And spotting can be harmless and just a nuisance or be a harbinger of significant disease.