Hmmm. The Pro-kits may have been a better choice. Sportlines drop you ~2 inches, maybe more. Too much for a Sentra. You just discovered how little compression travel our cars really have.
The Motivational shocks totally cure this. You only need to get new front shocks. In the back, their thinner shock mounts give you an inch more travel. I was quoted $325 each for new front shocks. Don't know about the rear mounts.