Well the signs and symptoms page says "more than 2 pounds in a week" http://www.preeclampsia.org/health-info ... d-symptoms
For me personally though, I can fluctuate a pound or two, especially if I eat a lot of salt (salt sensitive hypertensive here).
Though when I gained more than 5 pounds in a day, for me personally that was a sign of preeclampsia.
Just a thought, but is there any way you could go in to your docs and get a pressure check and protein dip to maybe help put your mind at ease? During my second pregnancy I was super paranoid (I mean hyper vigilant!) so my OB would let me come in whenever I wanted to get a urine dip and BP check. Probably since I only had to see the medical assistant for that!
I hope this helps, good luck!