Wow, I'm surprised no one responded to this sooner!
I too have had issues in the intimacy department because of back pain, and OY I feel you when there is just no desire to be found.
What has helped me is to try as many different positions as possible and find the one in the moment that doesn't suck. Sometimes its bent over the bed with my belly on the mattress and my legs on the ground, sometimes it's sideways on my back, sometimes there just isn't a way around it and I have to find another way to make the magic happen without actual intercourse.
I'm 35, and I know it's not easy when your mind tells you one thing and your body says another.
Oh, and if it helps at all, salonpas patches, aspercreme, or an icyhot comparative might bring a little relief.
Good luck, sista.
Arthritis issues since youth, back pains started at age 14 (1994). Dx with AS in January, 2016.
Other Dx: GERD, Raynauds, Bursitis in Left Shoulder and Right Knee