HI Albe,
You are not giving us much information to go on.

How long have you been riding bikes? How long are your typical rides? Are you young? Are you athletic? What kind of bike are you riding? Is this a new bike that puts you in a new riding postion from your old bike? Where is this numbness- on your behind or more towards the front? Knowing all that would be helpful to anyone trying to give helpful advice.

and Welcome to TE!