I keep waking up in the middle of the night after 4-5 hrs of sleep feeling short of breath, sharp abdominal pain, left shoulder pain. my watch showed segmented sleep pattern with nocturnal oxygen level 85%. is it caused by hypoxemia or gerd?

Hypoxemia is clearly suggested (providing reading is accurate) as one of the causes for your nocturnal symptoms. Pain by itself may contribute to sleep interruptions as well and will need to be tracked at its roots. Would first confirm the O2 sats with a reliable device. Involving your doc at this point will be best.

Could start an evaluation with an overnight pulse oximeter recording which could be done at home. Next, your might require overnight sleep study, looking at sleep apnea or other issues. Left shoulder pain should be evaluated as it too, is a disruptor.