I’ve not experienced this while cycling, but 2 years ago I used to do short 2 miles runs first thing in the morning 2-3 times per week. I would have to completely stop running and walk because my side would hurt so bad, and I was only doing a 9 minute per mile pace.
This didn’t feel like the side stitch that you get from breathing to me, it felt much much worse than any side stitch I had ever encountered. It also didn’t happen every time I ran. I ended up asking my doctor about it and his first question to me was about how hydrated I was.
I literally would wake up first thing and go out the door and start the run, completely dehydrated. Not sure about other people, but when I wake up in the morning, I am incredibly dehydrated.
I never tested the theory, because I ended up just quitting on running all together and focusing on cycling. But I have learned that hydration levels can effect so many things, I would not be surprised if it were the answer to my issue.