The answer is not straightforward, but there are a few possible causes. If you are new to inversion, it is possible that you are experiencing some soreness because your body is not used to being in an inverted position.
This is similar to starting any new exercise program – if you do too much too soon, you may feel some discomfort. Another potential cause of pain when returning upright is if you come up too fast. Inversion allows the vertebrae to separate and the discs to decompress, which reduces pressure on the nerves in the spinal column. When you return upright, the vertebrae go back to their normal position, and the pressure on the discs increases again. If you come up too quickly, you may put sudden pressure on the nerves, which can be painful.
To avoid this, it is recommended to invert to a mild angle (around 30-40 degrees) for a few minutes and then slowly come back up to a horizontal position, remaining there for several minutes to allow your spine to slowly re-compress. If you continue to experience extreme pain or discomfort, or if you always experience pain while inverting, it is important to stop and consult with a doctor.