I would recommend going to see your doctors
">primary care doctor as soon as possible. They will be able to help you figure out what is going on and also provide you with some medications that will help relieve the pain.
The most likely possibility is that you are suffering from a muscle spasm in the lower back. Although you are quite right that this usually occurs in the setting of a strain from lifting a heavy object or something similar, it can also sometimes occur spontaneously, for example in the setting of "sleeping funny." If this does turn out to be a muscle spasm, then your doctor may decide to give you a medication to relieve the spasm. Alternatively, standard anti inflammatories like ibuprofen may be all that is necessary.
You are also correct that back pain can sometimes be caused by other problems, such as a urine infection or a kidney stone. This is less likely in your case because you do note that you are urinating normally. However, your doctor will also consider these possibilities and, if necessary, may perform additional testing, such as simple blood or urine tests.
Schedule an appointment with your doctor's office today!