Sounds like a possible abdominal muscle strain. I'm currently nursing one that is similar to what you are describing.

Following, as I've been dealing with something similar since the spring. Lower right abdomen, when I stride out the right leg behind me, I feel a "pull" as if it's very tight, then usually goes away after a few minutes. Later in the day and the next day, it hurts to raise my leg.

Don't feel anything in 99% of daily life, and stopped running for a few weeks (for that and another issue). At PT ran 1-2 on the treadmill easy x 3 trying different things, that evening I can feel it already

Looking at getting imaging to try to see what is going on. If it were a pull I'd expect resolution after several weeks.

I had the same problem, took me two years to figure it out especially because this only occurred during racing, if you cant replicate the problem in training conditions its something you can figure out only very few times a year.

Turned out that its abdominal muscle cramps, especially those connected in the iliopsoas chain. Having an office job and sitting most of the day, those can be a little shortened in any case, as it was for me. In a race you have them tight and short sitting in the aero position for several hours, and then you switch to running and suddenly they are totally stretched from one minute to the other - no wonder those muscles go on strike =) Normally this should get better after 20/30 min in any case

I solved the problem with the help of my physiotherapist, lets of sessions during the winter going with stretching and relaxing those muscles, as well as daily stretching at home and in T2 (you will get strange looks, but stretching 30 seconds is much much better than walking half an hour). Also two brick sessions a week helped me with the bike/run transition, even if its only a 3km run after a long bike ride.

Seeing your problem is relatively new, and not a general problem like mine was, probably you can solve it with a short break to let the muscles relax and then focus on the stretching.

I've been doing something similar for the last couple of weeks on my own. I've been doing the "cobra" yoga pose for stretching, and some lower abdominal muscle activation work. It seems to be helping. Situps (getting out of bed in the AM), are less bothersome.

We will find out today, since I'm going to try a short run at lunch. Race next weekend, gotta keep some running in the legs.