Many people find their apnea is somewhat worse sleeping on their back. You might need a little more pressure to overcome this. Doctors usually set your CPAP pressure to give you good results while sleeping on your back.
Usually, sleeping on your back with CPAP is fine. Your AutoSet will tell you if it's not working, and you can adjust pressure or decide to try another position.
Get the free SleepyHead software here
for information on the main alternative to CPAP.
If it's midnight and a DME tells you it's dark outside, go and check it yourself.
I'm typically a side sleeper. With a constant pressure of 13 (my prescribed setting), based on looking at the data in the morning, if I roll onto my back, I get apnea events constantly every 4 breaths or so.
I'm curious why you suddenly feel more comfortable on your back now after being a side sleeper for so long. I have no trouble adjusting my pillows to accommodate my mask while lying on my side. In fact, I find that it helps _reduce_ leaks by keeping the mask pressed against the one side of my face, meaning that I only have to adjust the strap tension on the other side.