Tell her

"Mom! I made a mess!"

You were laying in bed, propped up on your hands with your crotch slightly above the sticky stain you had made in the night as your mom came in; leaning to the side a bit to let her see.

"Oh sweetie, that is a mess! You weren't breaking the rules by touching yourself, were you? I know you've been good for a few days now..." She gave you an accusatory glance as she placed her hand on your sheets to feel how "fresh" the stain was.

You denied it honestly, but you weren't sure if she believed you or not, so you let slip that you might have been dreaming...

"Oh, so you had a little wet dream, did you sweetie? That's nothing to be ashamed of, it's just part of being a boy. It also means that our little treatment is working! You're building up more testosterone and I think that with a few well-timed releases we can avoid more messes like these in the future. It looks like you were grinding on your bed for most of the night, so you must have REALLY enjoyed your dream! I'm proud of you for sticking with it so far Chris. Now get up and slip those panties off, I'll wash them in the sink while you strip your bed down and get those sheets to the washer. You should probably give your mattress a good scrubbing too!"

As she finished your mom's hand left the stain on your sheets and went to cup your face as she kissed you on the forehead. Her words slightly terrified you, since your nightmare had kind of made you sick to your stomach, but the massive wet spot under you surely betrayed you. Her kiss made you feel better though, as if you're little accident was no big deal, and you didn't even consider the sticky prints she left on the side of your face as a result; you just did as she said and got to work cleaning.