Guilt. Most animals masturbate, so it isn't sinful, it's biological. This a private activity for humans. But our culture has made it shameful, so we naturally feel sensitive about this and any other topic that is associated with sexual activity. Your parents might also feel uncomfortable around you, right after they had sex. Welcome to the post-victorian cultural remnants of humanity. You're normal.
Probably guilt. Depending on your age, maturity and preparation for sexual maturity, and depending upon moral, ethical and religious beliefs, one could have a sense of guilt after masturbating. Parental attitudes towards sex and sexual practices may influence your feelings of guilt. You should read more and inform yourself more on the practice of masturbation, its biological significance and make your own decis.