Other reasons for my acne?

Alright so acne is prone to pretty much everyone in my family including me sadly. I started getting small pimples when I was in third grade and I am now presently in seventh grade. My acne slowly started getting more severe. The thing is though is that it's not on my face. I do have some red spots on my forehead and some small black heads on my cheeks but other than that there is nothing. I started getting back acne when I was in fifth grade and it was really mild. During the time I didn't really doing anything about it I just thought it would go away on its own eventually. When I was in sixth grade my acne on my back gradually got even worse and it started to spread to other areas like my chest. The chest acne I first got was really mild and it would only be one or two small pimples on my lower and middle parts of my chest. THEN over the summer all of my chest acne was just gone, no scars, nothing and my back acne was clearing up like a lot. Then the week before school started everything just came back like literally over night. I don't know what happened. My chest acne is now horrible, it is all over my chest and it's red and same goes for my back. Most of the acne is now cystic which has never happened to me before. I have tried everything including, Clearasil, Clean and Clear, Neutrogena, Cetaphil, and Aloe Vera and even tea tree oil. So about three or four weeks ago my mom FINALLY took me too a dermatologist. The dermatologist prescribed me to take amoxicillin pills twice a day; once in the morning and once at night. He also prescribed me this tretinoin
cream to apply to my chest and back (and my face too if I really wanted too which I have) once a day at night. So far absolutely nothing has happened yet and my acne seems to only be getting worse than it ever has before. I am coming up to the prediction that the reason why my acne came back so forcefully at the time specifically is due to stress. I think my acne could be stress caused because I do have very low self esteem and I hate school and because my acne came back that week BEFORE school it makes me wonder. I also wonder if I am having severe acne because I have heard that dairy products can cause acne and I drink milk at least three or four times every single day. Help!

(Also, I have very good hygiene! I shower every day and I always wash my face twice a day!)