[Mako]

Very interesting question...
I am an altitude diver and I don't see any problem surfacing below the sea level. The difference should go towards safety and not increase risks of decompression sickness. What you should care about is what happened after surfacing. Are you going up a deep well using a lift? Even that should still be ok... as long as you don't ascend 1000m or the speed is still normal. ;-) If the cave is hermetically closed and you feel a depressure by opening it that could be a problem...
I believe more that you were dehydrated, staying in Siwa at this period of the year is very dehydrating... you can almost not drink fast enough what you sweat and the air is so dry that you don't even feel it. Your urine should be transparent, if it is yellow you should drink more! This is quite a challenge in Summer staying in Siwa! I had quite a few times guests on the boat in Egypt who had similar problems because they were dehydrated. Also exercise during and after the dive can be a cause try to get surface help to carry and help reduce your efforts.
But you did the right thing, Oxygen to a higher percentage as possible is the best you can do. I recommend 100% O2. Don't delay, the earlier you take it the better.
If you don't feel any pain now this means the O2 treatment was the right thing! Otherwise the pain will stay or increase.
Make sure you drink enough and definitely reduce your efforts!
If you still feel pins and needles or pain don't dive again and visit a hyperbaric doctor. Do the same if you experience again discomfort after the next dive.
If you have any more detail about the situation don't hesitate to write again.
The right procedure for a decompression sickness is 100% oxygene lying down during the transportation to the recompression chamber where the specialist can care for you.