Yep mate welcome to the old guys bit
I get something similar and have always thought of it as a sort of sciatica (trapped nerve)
I cure it by laying on my back and doing some stretches an guy taught me
Lay straight to start with and while keeping one leg straight bring the other knee up to your chest (holding it with your hands)
Then tip the knee over your body (so it would be left leg over the right hip)
Easier to show so have a look here
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You will know when you get it right it will pull at exactly the place that hurts
Amazing how soon it goes away when you get used to doing it
Oh and I have to say that since I started going to a Chiropractor regularly so many aches and pains have gone away so would recomend it to anyone
Just make sure you get a good one (recommendation is the best way)
Jason
Ps hope it gets better soon mate