Aren't there a couple of those half circle shaped cam seals back there near the up pipe too? Perhaps when you did the up pipe you knocked one loose. A broken or damaged cam seal would definitely cause you to leak oil in that area. From what I've read it will cause a pretty substantial leak as well. When you are under the car again look at your passenger side head near your uppipe and see if you can't see if one of those cam seals are pushed in or missing.
http://www.msmracing.com/images/STI_VII_LH_HEAD.jpg
For visual reference the seals can be seen on the right side of the head. They are little rubber half circles along the edge where the valve cover mates up to.