Don't be embarrassed at all. It's happens to most of us during puberty
Just make sure that you r more private in future

Don't say anything. Your mom will probably say nothing as well

It happens to lots of us, try not to be too embarrassed. I doubt she will say anything, and if she does, hopefully it will be just to ask you to keep your door closed in the future. If it was closed, you can ask her to knock first as you're growing up and need privacy at times.
Remember, masturbating is completely normal, for males and females of all ages, even those that are married or in a relationship masturbate, just not as often as teen age boys

Do not say anything. and don't worry. I understand the embarrassment but nothing happened that you need to worry about. Your mom knows that you are going through puberty and that something is changing in you and in the things you do. make sure you are discreet

If you dont make a big deal by saying anything about it your mother will not say anything to you.
Just nake sure you take care of your boners behind locked doors , that wsy there will not be any more embarrassing moments being walk in on

Well for what it is worth, she is likely embarrassed that she walked in on you in such a state. I wouldn't sweat it, despite how you feel. I reckon she probably won't say anything to you and you don't need to say anything back. We all do it, and those of us who deny it are probably lying as it's a natural thing for guys our age to do and she knows it. If it does come up in conversation, you have nothing to apologize for and like was said above, ask her to knock before entering your bedroom if the door is closed and if the door wasn't closed, then take care to prevent such an intrusion.

@collin13 you are 100% correct with your comment

why didn't she knock. She saw what she saw and should know better than to walk in a teenage boys room.

I got seen few times but it's nothing now.

Yes it's really embarrassing to be caught by either parent, especially your mom. Benn there and yeah it sucks. The thing is our parents know we masturbate, excessive tissues, crusty socks, sheets, long showers. I'm sure they also did it themselves as teens. Just let it go and as time goes by it will be forgotten.