I got it from swimming at a lake that is by my house. I forgot to mention that. If it does get bigger or starts to hurt worse, I will definetaly see a doctor.
I agree with the others it doesn't sound like chafing.
In your post you said it was a bump. Sounds like it could possibly be a boil. Sorry to be so graphic but does it look like a big pimple? If it is a boil you can apply a hot cloth on it and try to bring it to a head and it should erupt on it's on. If it doesn't clear up in a few days or gets bigger I advise you to get treatment by a Dr. The Dr will probably lance it ( make a small incision) to release the infection and give you antibiotics.
It doesn't really sound like chafing. That's usually more widespread.
If it is a bite of some kind, also put some bacitracin or other antibiotic cream on it as well so it doesn't get infected. An ice pack might ease some of the discomfort as well. If it looks like it's getting worse, though, see a doctor. My mom got a terrible spider bite one year in Ireland. It ended up infected and she let it go and it took quite a few rounds of antibiotics to knock the infection out.
Sounds more like a bite of some kind. Spider bites are notorious for that kind of reaction and most people never feel the actual bite or see the spider. Have you tried something like benadryl ointment or gel on it?