Agree with all the others, something has to be happening to make her back up. I would get saddle checked, maybe whatever position you sit in causes her pain when you ask to back up. I would get her teeth checked and make sure her bit is clean. Are you using a happy mouth? SO many of those rubber bits, the horse chews and they get little sharp ridges that cut into their tongue - that would make a horse back violently away. Get everything checked and next time, don't jump off, that's just teaching her to back up to get you off. Sit up straight and tall and pull 1 rein. Maybe you and your horse should learn the 1 rein stop, I'm fairly certain it'll work for going backwards too. Assuming she's balanced and won't fall over? Release pressure on the other rein ALL together - any contact may be driving her to back up.
The other thing - when a horse backs up on their own free will they will trot backwards. Not at high speeds, but they pick up diagonal pairs to move backwards. When a person is asking a horse to back up and the horse is unsure or nervous they'll stumble backwards one foot at a time. Ideally you want a horse to trot backwards - obviously only when you ASK them to back up though.
ETA: saddle fit changes too - whenever the horse has a fluctuation in weight or muscle tone/shape their back shape changes too, you need to continually check.