I would say that the gasket slipped while you were putting your pan back on. Get a new gasket. It would be best to use a gasket adhesive on the pan. It will aid in holding the gasket in place when you put the pan in place while you put the bolts back in. Also, don't over-tighten the bolts. They should be slightly tighter than snug. If you get them too tight, they will break your gasket. Get the gasket on correctly and fill it with the proper amount of fluid and you should be OK.
Hope this helps. Good Luck.