Lymph nodes do indeed occur throughout various areas of the body. They tend to be concentrated around major joints, such as the elbow, inside the armpit, the neck, the groin, the back of the knee, etc. If you have a bump in one of these locations, then it is possible that this is a lymph node. However if the bump is far away from a joint, such as on your stomach or back, or on your arm or leg, then it is not likely to be a lymph node. For example, if the bump on your arm is in your armpit or on the inside of your elbow, it could be a lymph node.
If you think it is a lymph node then it is worth watching for a few weeks. As you say, lymph nodes can swell in response to an infection, and this should go away after the infection clears up. However, if the node persists or is growing after a few weeks, then it is worth having your doctor
check it out.
Another possibility would be that this is a cyst of some kind, as cysts can grow in any location under the skin. Cysts are almost always benign, but the sometimes need to be removed. Talk to your primary care doctor