I can tell you from experience, nothing seems to feel good down there when I have my period. Cramping is common and can occur very low down in our pelvic area contributing to soreness. Also the whole vaginal area for many will become much more sensitive. Vaginal chemistry changes also. This means the PH of the vagina can be different and even allow for bacterial or yeast overgrowth. Aching is common and many can even get varicose veins around the vagina, especially on the labia majora (big outer lips). These will sometimes ache during menses. If this soreness occurs only with your period I doubt there is any need for concern. However, if symptoms are severe, or interfere with your lifestyle during this time, consult your healthcare provider.
As a mom, I’ve had many conversations with my daughters about how we each experience our periods differently. You are experiencing something similar to what I’ve experienced. Soreness, unfortunately, is just another by-product of having your period. With soreness I’ve also experienced cramps, pimples on my face, and bloating so sometimes it feels as if my entire body is changing for a few short days and then all is well again. If you are worried… first, check your pads. Are you wearing them too long? Are you wearing tight underwear or clothing? Next, make sure that the amount of blood you catch with your pad is a normal color. The color tends to run brownish toward the end but can be bright red during the heaviest flow time. If the soreness persists, please ask your doctor for an exam to check things out because I want you to be healthy and ease your mind about what’s going on with your vagina.
Does it feel like someone punched you in your vagina? Yes? Then, yep, that’s what happens to me too. Those are my least favorite days while on my period because not only am I sore, but out of either coincidence or bad luck, those are the days where I just so happen to run into corners of tables and/or desks. Ouch! Anyways, for those sore days, any low-dosage pain reliever usually does the trick. Or, you can always wait it out. Call me cliché, but “whatever doesn’t kill you, only makes you stronger.” I like saying that because makes me feel a little more epic for sucking it up most of the time. Given my lack of a medical degree and knowledge around the subject, I can’t say why exactly you’re experiencing this soreness in medical terms. See Michelle’s answer for that, or talk to your own healthcare provider.