Pain in lower abdomen and down under the vagina down the tailbone?

If I understand your question correctly, then you are having an inside pain from your pubic bone in the front around underneath to your tailbone.

Thinking about the site of your pain, it struck me that it is exactly the place where a muscle known as the pubococcygeus runs. The pubococcygeus is actually a subdivision of a larger muscle known as the levator ani. As you might guess from their names, the pubocoxxygeus runs from your pubic bone back to your tail bone (aka the coccyx) and the levator ani "keeps your anus off the floor." These muscles form the floor of the pelvis. They keep your organs up inside your body cavity and respond to pressure changes in your body cavity - for example, when you stand up or go to the bathroom.

So, I thought that you might be getting some sort of muscle spasms in the muscles, which would cause the initial pain, and make the muscles sore for some days, which would trigger the lingering pain when the muscles were stressed later.

When I searched for a site to give you as a reference for these things, I was surprised to discover that there is actually a "levator ani syndrome" that seems to perfectly match your description. See the bottom of the page at the link.

If you think the info at the link describes the affected muscles and your experiences, then feel free to show your mom and even to print out the part about levator ani syndrome to show your doctor. If you have seen an OB/GYN about this, I would probably expect that they would have thought of this syndrome. If it was a GP or a pediatrician, then maybe they wouldn't have heard of this.

BTW: the Kegel exercises that are mentioned at the link are contractions of the muscle more or less in the same way you squeeze to stop the flow of urine. Next time you have this pain, as the link suggests you can try doing that to see if it helps.

HTH and that you can get some relief. BTW: since we cannot vote for best answers anymore, I would appreciate it if you could pick a best answer for your question. I think you get points back for choosing a best answer.

Normal for mensus.

you may apply hot water packs to the lower stomach and lower back to reduce the pain.

There's nothing you really can do besides keep going to doctors until someone takes you seriously. (Go to a different doctor and get a second opinion) Or next time this happens go to the emergency room maybe you will be taken seriously there.