Its important to keep your cool. Freaking out won't help you in this situation. Just wait to see what she says, and if she starts freaking out, be the calm one of the both of you. If you don't freak out too, then she will have to come down from her freakout to talk to you. Just keep calm. She probably won't have a problem with it if you calmly explain to her what it is all about.
Now from a mother's perspective, just putting myself in your mother's shoes for a second, if she didn't know you had all of these exes and your mate, then she might freak out about that. Its natural. But again, if you stay calm she will have to calm down to talk to you.
And who knows, it could be she could just hand your your phone back and say, "nothin weird on here

"
Best case scenario

other than her not looking through your phone.