Legs shaking when receiving oral

So I've posted a few questions about my situation, my boyfriend is unable to make me orgasm through any sort of penetration/oral/foreplay etc. We have been having sex for a while now and I can never orgasm with him and he is well aware of that. I can orgasm by myself watching porn and using a vibrator on my clit. Anyways, yesterday he went down on me and licked my clit, nothing mind blowing was happening but my legs started to shake. I know what an orgasm feels like and I know that at the time I wasn't even close to climaxing, so why were my legs shaking?