I am very much a noob when it comes to skydiving, so I'm no real help in regards to the leg strap issue.
However, what you are describing is something similar that I went through a few years back where sudden changes in body position caused me to faint. Orthostatic hypotension is something you may be familiar with. If you lie on a couch and then jump up to, say, answer the phone, you might get black spots in your vision or briefly black out entirely. I'm unsure if you've ever fainted before (?) but seeing stars and feeling nauseated are all part of that. It sounds similar to my own hypotension issues which were easily remedied. Definitely see your doctor, posthaste!
And congrats on passing your jump!