Lower back pain causing tailbone to go numb

A little back story. 2 years ago I went through martial arts course for the Marine Corps which involves low crawling and bear crawling with upwards of 80-100 lbs on my back. Shortly after about once every two months my lower back would pretty much give out on me and I’d have to sit down or hold onto something to stay standing. After about ten minutes it would go away and it wouldnt happen again for another two months or so.

Fast forward to about a month ago and it’s been happening at least once a week sometimes twice a week. My lower back will give out and tighten up on me and I won’t be able to stand. Shortly after my tailbone will go numb and feel tingly.

Other info that might help
23 years old
4 and a half years in the Marine Corps infantry (plenty of long hikes with heavy weight)
I work out in the gym every day well until this starts happening and I just shut my workouts down.

If you know military medicine it’s not the best, how serious is this and has anyone else here had this problem before? What was the solution given by the doctor?