You really don't have anything to worry about since at your age, periods can be irregular. You can talk to a close friend or your school counselor or nurse about it. They will be able to help if you are worried.
It's all right dear. Periods are not always regular especially when you are 15. I'm in my 30's and mine sre not always regular. I'm not sure why you can't talk to your mom but my guess is because you are sexually active. Please forgive me if this is not the case because you certainly did not say you were. I'm just trying to figure out why you can't talk to your mom. If you happen to be taking the pill this can also cause your periods to be irregular. Talk to a friends mom, a friend, a friends older sister, a teacher, counselor, school nurse etc... if you are upset. Ok everyone. Don't get to emotional that I talked about the pill to a 15 year old. I believe in teaching young girls abstinence. But, in the real world I also offer education and prevention.