Did you add oil to the gas when you filled it up? Is the bike new to you? Two stroke engines use oil in the gas for lubrication, unless they are oil injected that is. In which case you gotta keep the oil tank full.
If the problem truly happend after adding gas I imagine you diddnt add oil and blew up the engine.
Otherwise, maybe your exhaust just started leaking by coincedance. Grab a wrench