Why is there fuel in crankcase of 2003 rm 125?

dont start it without changing the oil. I would check to see if the carb needs cleaned .the needle and seat could be hung open with dirt or something. If so it can fill the motor with fuel.Did you leave the fuel on,off and reserve switch on for to long when not riding. well change oil then check for compression.the top end could be letting the fuel mixture blowby.