A lot of people get soft in their core during burpees during that phase, resulting in excessive lumbar extension. If that's the case, then it could be your facet joints, anterior spinal ligament, anterior disc, nerve (if you got any pain into the leg, numbness, tingling, searing). Take it easy for a bit, you may have just irritated the joints. Currently, stay away from burpees and when you go back into them later (maybe in the next 4-6 weeks if you feel alright) make SURE you aren't caving in your low back into extension. Stretch your hip flexors a lot - if they're tight, they're going to pre-position you in extension and bringing the legs back will make it worse.