Pretty sure tail bone is the problem. Get a mat. And if you're doing 'em correctly, you should be worried about that painful, yet orgasmic feeling in your abs. Like sugary delights being pumped out of a factory run by little 6 year old Chinese kids.
The exercise does actually work your back though so you'll get used to it. Try some heavy deads and squats, that will help build your lower back strength alot.
if u didnt feel anything then try use a heavier weight. Im usually quite numb after doing this, and the next day i will be in pain (DOMS - delayed onset muscle soreness).
the reason its hard to keep ur arm straight is because holding a heavy weight at arms length away from ur body means it will tend to sway to the side, your natural reaction will be to straighten it up, this requires other muscles along ur arm/forearm, which is why it seems hard.
its even worse when ur using 17.5kg and ur doing it in ur backyard and its windy