At least we are taught at college that generally internal exams aren't done on virgins unless there are very good indications or it's needed for forensic reasons. This is because it would be painful and uncomfortable and if you are not sexually active there aren't many problems that can be seen by doing an internal. But if it is required they have smaller speculums.
But I would be surprised if they wanted to do an internal for PMS without other specific symptoms - they might do bloods or ultrasounds but they aren't very intrusive.
And just remember, you are the patient - if the dr wants to do something you are uncomfortable with it is your right to refuse. The dr should give an explanation as to why it is important but even after that it is still your decision, you are in control. If you refuse he will just document that it was recommended and you refused to cover himself for legal reasons. It is also your decision if you want your mom there, but drs often want a chaperone with them during the exam - often a female nurse to protect themselves and give you a hand to hold.

You are doing the right thing. And drs know that adolescent girls can be very embarrassed and modest and they should make you feel comfortable.