Girlfriend smokes weed behind my back I think. Help please


I have a tricky situation... I have dated an amazing women that I love and when I met her I knew she smoked weed. I never had an issue with it. I don't smoke or do drugs at all. I am a clean guy. I thought it would not bother me if she does it but when I first saw her doing it it broke my heart ( my family member died from smoking ) So when she goes and does it with her friends I have a really bad feeling in my heart. I love and care about her and I worry. I got really upset the last time she did it and I was honest with her and said it really bothers and hurts me that she does it. She said that she loves me and that she wold quit doing it.

I just have a feeling that when she goes out with her friends and I am not there that she will do it behind my back event though she said she would quit as she does not need it in her life.

I am afraid that when she comes home she will smell like it and I will get upset. I don't want her to lie to me cause this may make me not trust her,

My question is why does it bother me so much and should I really care that she does it when I am not there.