It can actually be quite a few different things from your description.
What parts hurt? One of the (or multiple) joints (shoulder, elbow, wrist)? Or just a spoton your upper arm or forearm?
If it's just a hard spot that's sore, it could be a reaction to a spider or insect bite. Sometimes such spots would be red and feel somewhat warmer than the rest of your arm. It won't necessarily itch. Did it start hurting when you woke up one morning? Often, spider bites happen at night. They're typically not serious and should go away in a few days.
It could also be inflammation around a small fracture in one of your bones. That would hurt and swell without necessarily being a huge impairment on your ability to use the arm. Can you trace the pain back to hitting your arm on something, or bracing yourself on a fall? If you think this could be the case, you should see a doctor. In the meantime, you should take it easy on that arm.
If the pain and swelling is in a joint or multiple joints, there are lots more possibilities. Sprains can cause swelling that can be hard. There are other things it could be, though most likely an injury like this.
I'd advise putting an icepack (can be as simple as ice in a ziploc, outside your shirt) on it and see if that helps. But if the pain is intense and doesn't get better within a day or two more, definitely let your mom know what kind of pain you're in and tell her you really think you need to see a doctor.
Hopefully it's nothing, and you'll be right as rain in a day or two!