Oil leaking from driveshaft!!!

[lightsdarkness]

I have a ninja 250 that, I bought for my friends to learn how to ride on and now its my gf's turn but I have a problem now.

I adjusted the valves about 3 weeks ago and then went for an aggresive ride after the adjustment. I put the bike away and after two weeks of it sitting it started leaking oil. It is leaking oil from the driveshaft. When I took off the front sproket I could clearly see that it was leaking from there. It leaks about a 1/2 a cup a minute when running. WHat can I do to fix this, I have never encountered this before. Any help would be great guys!