Sore Hip Flexor

I am looking for some stretching or perhaps technique advice for a sore hip flexor. I usually swim 4 days per week in a pool. A few days in a coached workout and a couple on my own. I also do some dryland/strength training and I walk. I have a bad hip (pins and osteoarthritis) so I'm limited in what I can do exercise-wise. My hip-related soreness is usually pretty limited while I swim and only on the side of my bad hip. I only compete in open water and only get to practice in the open water every few weeks. The past few times I've done open water swims (just practices so I haven't really been swimming all that hard), I've notices some tightness/soreness in the hip flexor of my good leg. In fact, the last time out I ended up doing a bit of one leg kicking with my bad leg just to give my "good" leg a break. I was able to stand up for about 10 seconds in a shallow area and just brief break was enough for me not to feel as much discomfort in the area for the rest of the swim (I was almost done with my 2 miles by that point). Anyway, I have no hip flexor discomfort during anything else that I do including my regular swim workouts. I suppose that even though I swim harder during my regular workouts, I am getting my 10 or so seconds of rest on the wall between swims. I do stretch but I thought maybe someone might have stretch or technique that I have not tried. I have a couple more open water races coming up and I don't want to be uncomfortable. Also, I have no residual soreness after the swim. It's only while I'm in the water. The first open water I did this season, it was sore almost from the beginning. The one mile swim I did a few weeks ago it was only sore near the end. And the most recent 2 miles I did it was sore probably about 2/3 of the way through until the end. I usually do between 2500 and 3500 during my swims depending on how much time I have to swim. Aside from the hip flexor issues, two miles in the open water does not feel hard at all. Thanks for the help.