In recent weeks, having hypnic jerks in multiple muscles almost every nite (used to be rare for me). don't feel like i am falling nor am i almost asleep, just very relaxed. what could be causing them?

No way to tell: One of the things that this pathologist / blood donor has discovered is that curious things happen as your body concentration of iron changes. I noticed you are are having yours replenished, and the myoclonic jerks -- which many people experience as pleasant once they get familiar with them -- may be a sign of your return to full metabolic strength. These rarely indicate serious illness.

Few things : Stress, sleep deprivation, caffeine, excessive exercise and some meds can make hypnic jerks worse.