How do I prove that I'm not lying?

I have a tumor on my spine so I take pain meds. My little sister is dying from Leukemia and I have to drive about 12 hours just to see her. The trip has been really tough. It was my understanding that when you sign a contract with a pain mgmt doc that you can still get meds from the ER. Since I've had to drive a lot with my sister my pain is worse and I've been getting bad headaches. I've been to the ER several times. So I go to the ER and I get a prescription. I was shocked when I got a prescription for #15 10mg percocets and flexeril. I got it legitimately through the ER doctor. Now this doctor is saying that he didn't write the prescription. What is even more crazy is that the doctor to which the prescription was written from was a doctor that was 2 hours from where I got the prescription. I'm being told that this doctor isn't an ER doctor but rather an oncologist. Let me remind you that my sister lives 12 hours away. I've never even been to the city that the doctor practices in. I might think I were just plain crazy except my husband was with me. Now my pain mgmt doctor is going to fire me and my record is tarnished. What should I do?