Even most outgoing persons sometimes find themselves shy in certain situations. The more we don't know about our whereabouts and be less knowledged about situation we are shun to go through. So, my friend some levels of this disorder(actually not an disorder)is something very normal. But about telling your mum, i think just tell her same exact words your typed here! I think that'd do just fine, it will be a proper first step. If you are not ready you can start with friends and talking to us!