Newly Diagnosed with AS

Congratulations on the baby! I am so sorry that you are having so much pain. I see you have UC so that rules out NSAIDs. Does Tylenol help at all with the pain? I know that heat can really help me with some rough patches. I have an oversized heating pad that gives me great coverage on my lower back and hips - all at one. I also use pillows to "brace" me when I am laying on my side. These are dense foam that I can lay up against. One of them I bought at a maternity store.
One of the things most I know with AS notice is that the back pain lessens once we get moving. So your exercising is good. I find that if I sit still for any length of time during the day, my stiffness and pain return and I have to "loosen" up all over again. So if it's necessary to sit for any length of time, I make sure I get up and walk around every 10 minutes or so.
As far as the waking up in the middle of the night with back pain - I did that for many years. I sometimes needed to sleep in a recliner to get a decent nights sleep. I also got used to getting up and walking around for 30-60 minutes in hopes of the pain lessening enough to allow me to return to sleep. We finally bought a memory foam mattress pad topper, 3" thick. That really helped get me out of the recliner and back into the bed.
Pregnancy also can cause back pain so knowing what is going to work might be tough to sort out. I hope you can find some relief. Let us know how it's going!