Hi Lizzie
I used to ride as a kid, however I didn't ride as serious as you...
I was sent airboune and cleared the jump, on a horses refusal at it ... ended up hurting my back on the bar.
Sounds like you've pulled or torn a muscle, or even trapped a nerve, are you getting any pains down the back of your legs?
It's best to get this seen by your doctor..... and get refered to a back specialist at a hospital.
A doctor will prescibe anti-inflamatory and pain killers if needed, which does relieve the pain , but doesn't cure the problem.
Last year I had a disc removed from my spine, and still seeking further treatment .... so get it seen to sooner rather than later.
Do you still ride? as I know you have to protect your back (as it is with you thoughout life) have you considered using a 'girdle' back support whilst training to give your back the support you need, also, underneath your joppers, make sure you wear knee supports as well, as these are the joints that will also suffer whilst riding. Also do gentle floor excersises to strengthen your back and stomach muscles, as these muscles protect your back..... before getting on a horse, like doing arobics, or any work out, you need to warm up first.
Doing 'sitting trot' doesn't do the back any good if you bump back down in the saddle to hard.
Hope you feel better soon, and get that back well rested before you get back on your tall friend.
Angie (The Ginger 1)