Huh. Oh well.
It's not really connected to your activity level; it's connected to the ongoing negotiations between your immune system and the placenta over the amount of blood ferried to the placenta, and the available nutrients and oxygen in that blood. Your body wants to restrict this (possibly because the baby won't fit out, possibly because there's a hard metabolic limit to the rate at which your mitochondria can convert food and muscle tissue into energy) and the placenta always wants more than you want to send. In preeclampsia the negotiations have begun to break down.
When that happens pressures turn labile, and oscillate between extremes. So readings that swing rapidly are something we expect to see as symptoms progress.
If you take your pressures from the same arm more than once in several minutes - I want to say 10, but can't remember how many minutes exactly - then the residual compression from the first reading will create errors in the second reading. So don't do that.
Are you logging your pressures for your docs?