That could come from too many straight jabs. Start dipping, bobbing and weaving. The dip is effective after a bob or weave. Will keep your opponent guessing. When a punch pushes your neck back, it causes your spine pain. You can also sit at the edge of the ring with your neck hanging forward and lift it up. Then lie on your side and bring it up and of course switch. That is a good way to strengthen your neck.
Stop getting punched... No seriously though, it's not the jabs guys, it sounds like you're too tense and hunched over when in your fighting stance, try relaxing a bit more, as well as stretching and warming up your back properly.
I recommend strengthening the muscles of your upper back and shoulder blades to prevent soreness. You're looking to develop muscle endurance so set of 20 to 30 repetitions are key.
Make sure you stretch out your neck before sparring. Your neck has muscles in it just like your arms and legs, but most people never stretch or try to strengthen them. Tilt your head forward, back, and to both sides for 10 seconds each. Push on your head to apply extra pressure if you aren't feeling the stretch of your neck muscles. Turn your head as far as you can to the left and hold for 10, then turn it the other way and go as far as you can to the right for 10.
This^ If you are too tense and crouch your head and back this could certainly be the cause from the stress you are placing on your body. Make sure you are fairly loose
my back used to hurt boxing and i realised/was told it was bad form. When moving ect slipping weaving w/e i was moving my upper body but not bending my knees and using my feet and i was hunched over. Once i really thought about this my back stopped aching!
