The pic looks like the bumper cover has come loose at the fastener point (red arrow part). I had similar problem with Civic bumper cover, but could never fix it without replacing entire cover. Yours looks like it can be fastened back into place though. I am assuming this is not PRIME. You might ask mod to move the thread to Gen3 forum so you get more insightful helps from people who knows your car better.
Yeah, I saw the title and thought, "WHAT?! A Prime with saggy bottom already?" Anyway, my PiP had the same thing going on. I would periodically push it back in place and it would stay for a little bit and pop out again. It had enough other blemishes that I never got serious about fixing it.
