Congrats on the AG win! At my marathon, I noticed a side stitch (not nearly like the cramp you mention) only on the downhills too. What I did was switch my breathing so that I was exhaling when the opposite foot hit for a while. Eventually it went away, but of course that wasn't at 5K pace. But maybe it's a similar thing, just more intense because of the faster pace?
Congrats on the race.
About the returning side stitches: last fall in my half marathon, I got side stitches. I did not slow down, tried to run through them. Then I had them on both sides at the same time. I still tried to run through them, and eventually had to slow down as there was no way I was going to finish in that kind of pain. For maybe two weeks after the race, even easy pace effort would bring back the stitches. It seems like I had hurt something inside and it was not healed. Maybe something similar happened to you: you ran through the stitches, but afterwards, even easy effort brought them back.
Sorry it didn't go as well as planned, HCK, but congrats on the AG win.