I'm sorry to hear about your back pain. Based on your description, it sounds like you may have strained your lower back muscles during your deadlifting session, and the subsequent physical activity may have exacerbated the injury. Here are a few possibilities for what you might be experiencing:

Muscle Strain: Lifting with poor form can cause muscle strains or tears. This is common in the lower back when performing deadlifts incorrectly.
Ligament or Tendon Injury: The strain could also involve ligaments or tendons in your lower back, especially if the pain intensified with continued physical activity.
Disc Injury: Although less common, improper lifting can sometimes lead to disc injuries, such as a herniated disc. This can cause significant pain and may radiate to other parts of your body.
Immediate Steps to Take:
Rest: Avoid any activities that could further strain your back.
Ice/Heat: Apply ice to reduce inflammation for the first 48 hours, then switch to heat to help relax muscles and improve blood flow.
Pain Relief: Over-the-counter pain medications like ibuprofen or acetaminophen can help manage pain and reduce inflammation.
Gentle Stretching: Once the initial acute pain decreases, gentle stretching and mobility exercises can help. Avoid any movements that cause pain.
When to Seek Medical Attention:
Persistent Pain: If the pain does not improve significantly in a few days or worsens.
Severe Pain: If the pain is severe or debilitating.
Radiating Pain: If the pain radiates to your legs or is accompanied by numbness or tingling.
Loss of Function: If you have difficulty standing, walking, or performing everyday activities.
It's important to listen to your body and not push through the pain. If your symptoms persist, it would be wise to consult with a healthcare professional, such as a doctor or a physical therapist, to get a proper diagnosis and treatment plan.