This could be from many reasons, including a cut or abrasion, an infection (including an STD) or something like yeast or bacterial vaginitis, or a lack of lubrication.
Here are a couple of links that may help, but ultimately, you'll need to be seen by a doctor to get diagnosed. Please don't wait. A lot of infections left untreated can lead to long term problems, including serious pelvic infections and infertility.
Let us know what happens!