Obviously nobody can diagnose injuries based on Lemmy comments. My advice is to find a good physical therapist. I found one who specializes in my sport (powerlifting) and he fixed a nagging injury I’d had for years.

Hot packs might help discomfort but they won’t help heal anything. Warming up is certainly important. Stretching could be important but the hard part is figuring out what you specifically need to stretch, which is where a physical therapist can help.

Running shoes can definitely be the problem. It’s a layer of squishy-ness between you and the ground, which counteracts the taut-ness your body will get from good form. There are tons of options for shoes that have solid, non-squishy soles, but I prefer to just go barefoot.

I’m running, doing core and arm workouts and eating well, and losing weight - but I can’t seem to shift the belly fat.

Just to be unambiguous, the only thing really affecting your amount of belly fat is the amount of fat you lose. You can’t spot reduce fat by doing core workouts. It’s good for general health and fitness to work out, but training abs won’t decrease belly fat at all - you need the calorie deficit for that (which you apparently do have, so that’s good, and you should work out to make sure you don’t lose 80% muscle weight but mostly fat).

Where you lose fat just depends on genetics. For me belly fat is some of the last fat to go when losing weight and some of the first to come back when gaining - results really tend to come in at the end of a cut.

dropped 4 kg, and gone from a 34 to a tight 32 on my pants (trousers)

Sounds like there’s progress. If you want more, continue.

When we’re at our biggest, our skin holds everything “up.” When there’s less everything to hold, our skin sags lower due to gravity and our fat hangs a bit lower. This is why the spare tire “collects” around the lower part of the abdomen when everything used to hang higher when we were at our biggest weight.

You probably have lost some girth there in the middle. Try your non-stretchy pants that you used to wear for formal occasions and see if they fit any differently now that you have lost 4 kg. Or, an old leather belt with well-worn belt holes can show progress because we’re suddenly needing the next smallest hole to hold our pants up.

I would try getting up and walking around every 15 minutes or so if you can. If that doesn’t work or you can’t, I’d see a doctor or physical therapist. Diagnosing pain from a comment on the Internet is a recipe for a bad time.