Other people who care about us are going to ask/tell/speak to us about what they see and feel. I think the only way you can get others "off your back" is to either change or not be around those people. You can't hold your liquor that well if someone is suspicious and asks if you've been drinking; it's generally a rhetorical question at that point.

There are no "reasons" to drink other than you enjoy it and it doesn't interfere with your health or life. If other people comment, it's interfering because they are interfering. Your mother probably has better things to do than worry about her 21 year old daughter, so deal with your problems in a more healthy way and help her out.