Why am I getting billed from my ER visit ?

Tricare is an HMO it's not magic, it doesn't know who to pay if you don't give that information. It's only automatically covered on post and with network providers, and even that isn't unlimited.

You've learned a valuable lesson about health insurance and why people join the Reserves after active duty for health insurance. You're very lucky, this could have been a much more expensive lesson.

You need to get into the Tricare website to see what they do and don't cover, especially while traveling (like when you're on leave). You're required to contact Tricare and inform them when you seek emergency care off post within a day or two of being released. You need to do that now so that Tricare can cover your ER visit. Be aware that Tricare has caps on certain expenses and you might still be liable for some of the charges.

Whether or not You end up paying all or part is going to depend somewhat on the Line-of-Duty investigation of Your accident. Did You try to get the ambulance to go to Womack? JAG should be one of your upcoming appointments.

Unless you needed a Level 1 Trauma Center, most EMS agencies would have taken you to the local base/post if you had asked. Secondly, most medical policies (including Tricare) won't pay a dime until after the vehicle policy has paid. As this was a vehicle crash, in most jurisdictions, the vehicle insurer is on the hook for any expense up to the amount of the policy. Worst case, you will have to retain a competent attorney and sue for any unpaid claims assuming you were not at fault.