My mom cried when she first found out about me as well... To be honest, I wish I would have listened to her back then when she told me I didn't need it. It started off very recreational for me (only on weekends, parties, etc.) but once I moved out, it slowly increased until I was smoking everyday. Took me about 3 years of smoking multiple times a day to realize it was terrible for my health, and 2 more years for me to quit. I'm only 27 days clean but I can tell you it's no different once you get over the psychological/physical dependencies. I can still relax and chill, I just find other means. Growing is still a hobby for me though!
The most important thing is for you to decide for yourself if you really want to smoke & the pros & cons of that choice, it has to be your choice. (The above comment could be you years down the road or not).
If you trust your mom & if her reactions or what she had to say made you feel like you should quit, then I would follow your instinct, because maybe then you really were looking for a reason to stop, because you aren't ready for it on one level or another to accept it yourself. Whatever choice you make will be the right choice, if you can live with it. MJ use is an individual thing, depending upon the user & their individual lifestyle, morals, beliefs, & many other things. For some people it can be good, even great, & for others it can be not so good.
Don't bother buying mom anything, that is like a bribe or asking for forgiveness with a gift & in this case I think highly inappropriate. For someone who is truly upset or against you doing such a thing, a gift I think would be highly upsetting & insulting. A parent wants only one thing for you & your life, the very best you can make it, for you to be healthy, safe, & happy. And obviously MJ use is going to stir up all those emotions from mom, and questions...are you hurting yourself, are you going to get caught & go to jail, are you doing other drugs, what else doesn't she know about her sweet baby boy etc.
The gift you need to give her is REASSURANCE. Let her know, that she knows YOU. Clarify that you are not doing other drugs if that is the case. APOLOGIZE profusely about your disrespect in the fact you were smoking in her home & you had no right to do that without discussing it with her first, period. You had no right to subject her to your usage & possible ramifications from such (gettting busted) in her home, thereby putting her at risk. Let her know you will NEVER smoke again in her home, unless she is okay with that, & MEAN IT. Other then that, you are an adult, it has to be your choice, but mom need not know about your usage & shouldn't know because ADULTS keep their private business, PRIVATE. Tell her you love her & you are sorry & promise she has nothing to worry about & ask her if you can both pretend you didn't make the mistake you made by smoking at home, because you are the SAME person you were before she saw you doing that. And finally tell her, her opinions, feelings & advice are still very important and you will always truly take anything she has to say to heart & really consider what she has said. If she didn't tell you, ask her directly what are her specific concerns, & then address them indivdually & answer her honestly.
I think you will be just fine, & it sounds like she did a great job raising you, for you to reach out like you did about this issue in your life and to want to make her feel better and that you care that much about mom.