I get head rushes and dizziness when i stand up a lot... what is happening to me?

Dizziness: Several possibilities. Check your Blood pressure & contact your PCP.

Most likely lower BP: & BP drops when standing rapidly resulting in drop of blood flow to brain. This is rarely a health problem unless you fall and hurt something. Best be more aware & stand (especially from supine position) a bit more slowly giving you brain a bit more time to tighten up the muscles in your veins & ↑ heart contractility/rate slightly to compensate & help prevent the drop in blood flow to your brain.