The presence of numbness suggests that there is pressure on the nerve that serves that area. However, that can occur with virtually any cause of lower back pain, so does not help differentiate the cause of the back pain.
The most common cause of lower back pain after lifting a heavy object is a muscle strain. This can lead to significant muscle spasm and tightness that presses on the nerve after it leaves the spine.
It is also true that there can be other conditions within the spine that presses on the nerve, such as a herniated disc or arthritis, but these conditions would be less likely at this point. If the symptom persists, these conditions would be more of a concern.
The usual approach would include resumption of the muscle relaxer, along with an over the counter anti-inflammatory medicine, such as ibuprofen or naproxen. It also may help to apply local care, including ice, moist heat, massage, and gentle stretching.
If the pain becomes more severe or if it becomes associated with weakness of the leg, that would be more worrisome and would warrant being seen more quickly. If the current symptoms persist despite home interventions
, that also would be an indication for being seen.