I can't say for certain if what you're experiencing is an orgasm, but what you're describing sure sounds like one. That build-up of tension followed by a release where you have muscle contractions is what many people describe orgasms as feeling like. Usually, those physical responses are accompanied by pleasure. If you're curious, this article goes into more detail about what orgasm involves and feels like: Sexual Response & Orgasm: A Users Guide
. Does any of that help clarify things?