Oil Mystery

So I have my '08 450 for a few months now and no issues. Did the cam mod, K & N filter, rejet, changed oil a couple of times and so on. A couple of weeks ago we are riding and I noticed a bunch of oil on the left handside near the front sprocket area but couldn't really tell where it was coming from because by that point the bike was dusty and dirty and oil was everywhere. I checked oil level and it was low so I decided to add more and keep riding until I get back home to check on it after I have it all cleaned up. I had to add oil a couple of times while still riding. So I get home clean it up really good, add oil again and put it on a stand and let it run for at least 10 minutes shifting up and down. Not a single drop of oil anywhere came out of it. So I checked oil level again and when I unscrewed the dipstick, oil was all the way to the top of the reservor and some even came out once I removed the dipstick. At that point I drained all the oil from reservor, crank, and removed oil filter but have not gone back to try again. I need to buy an oil filter and replace but wonder what could have caused this problem. Anyone have any ideas. I have never seen this before.