No it's not impossible. There are medical conditions that cause a woman to have a period while pregnant. one of such is a double uterus. So no it's not impossible.
It is impossible to be pregnant and have a period because of what the two entail. It could be vaginal bleeding, which can be quite similar to a period, but if it's what appears to be normal to her, then there's little need to worry. The only way you're going to know anything for sure is to take a test in a few weeks. Until then, relax. Stressing isn't going to change anything, and will just make life harder for you.
So if she's having her period right now, should I still worry? Or am I just looking into it too much.
No, symptoms would not show up within 4 days. It takes at least 2 weeks for the fertilized egg to even attach to the uterine lining. Which is when it's possible to get a positive pregnancy test and most women don't get symptoms until 4 weeks or later.
If any semen (or pre ejaculate) got near the vaginal opening and she was ovulating she could be pregnant.
I guess a big part of my question is if symptoms could show up within four days. Doesn't it usually take about a week? And given the circumstances, are the chances of pregnancy very low or not?
Most pregnancy symptoms are the same as Menstrual symptoms, usually the cramping isn't as severe, and the bleeding isn't as heavy most of the time, however it is possible for women to get what appears to be their periods the entire time they are pregnant, so the best bet for you guys is to do a home test and see a doctor if the cycles don't become normal. Also sperm can live for up to five days under the right conditions so it is possible that by you just rubbing along the outside of her vagina that she could still become pregnant.
Depending on age, it's not unheard of that it could be early. Especially if the last one was early as well. A period is triggered by ovulation, so if she started early one cycle, she could have ovulated early as well and then had a second early cycle. She can take a pregnancy test in about 2-4 weeks with first morning urine.
It is possible for some women to mistake a period as implantation bleeding, but implantation bleeding is also light, and generally will last a few hours to a few days. It shouldn't require any form of pads or tampons. You don't seem as though you're actively trying for a baby, so I'd suggest taking more preventative measures in the future since these can cause pregnancy. Try to see if she can go on some form of birth control, since these can easily lead to actual sex. If she doesn't want to, make sure you have condoms and use them before you ever get close to the vaginal area.