My mum is annoyed that we're finding out the sex of our baby.

We are going for a gender scan tomorrow (baby #2, didn't find out with our son). I told my mum and asked her if she wanted me to tell her baby's sex once we find out. She angrily said "no - you should just wait - waste of money" and now I'm really upset about it. My boyfriend wants to tell his family - but I feel uncomfortable with his family knowing when mine don't. I feel like my mum is spoiling this for us - we were super excited about finding out and now I don't know what to do. Has anyone else been in a similar situation?