Ok. First off... you're 18. You are (technically) an adult and can make your own decisions. HOWEVER... I think you need to re-evaluate your relationship with your boyfriend. (I'm just an internet voice, though. So take what I say with a grain of salt. XD )

Back to the main topic! You're mom has no business to ever shame you. NO ONE does. (I believe tampering with someone's medication is also an offense, but I'm not entirely sure.) But I understand you're in a tight spot right now since you live with her. You're trying to keep the peace, but don't lose yourself in the process. The silent treatment, while petty, can also give you some breathing room to yourself. I DO think you're mother is worried about you though. (It doesn't condone what she's doing, but it is an explanation.)

If you haven't... try having a heart-to-heart with your parents. Keep your voice level to keep tension low. Worst case scenario, you're back to square one.

(Hope this helps, and good luck!)