@jennixxx I think this could end up being PGP possibly - I also have a weird stomach tightness/pain which comes on randomly when walking or exercising and agonising lower back pain. Mine doesn't radiate down my legs/thigh but I know that's very typical of PGP. I got a physio referral and have 3-weekly physio sessions where she massages me to loosen up the muscles. I also have an elastic pregnancy belt to wear when walking, and have a series of exercises to do including child's pose, side-child's pose, cat/cow, and something called a cradle which is basically rocking gentle side to side in tabletop. I've also got a pregnancy pillow to keep my hips/legs in line when sleeping, and under my side to lift my bump too - pillows can be used instead to do this. All of these have helped but still some days it's just unavoidable. Definitely get your midwife to check you out just in case but if it is this it's nothing scary, just annoying and painful. Apparently it goes away after the baby is born!
Also she recommended to really work on pelvic floor exercises as this can apparently help. I have to do 10x 1 second pulses and 10x longer 10 second holds 3-5 times a day (or whenever I remember)
Hope this helps - at the very least you can implement some of these things quite easily while waiting to hear back and see if they make a difference! Good luck :)