What do I do 😩

So I’ve been wearing ‘proper’ bras for like 2,3 years and I’m a D but I keep popping out of them so I tried different styles but it keeps happening and it really hurts and u can c it through my shirt Evan at school and it means that I always have to wear a coat so u can’t c but my mum keeps saying it’s cas it’s that time of the month for about 3 mounts and won’t buy me a bigger size cas my sisters weren’t a size D at my age WHAT DO I DO !