laying on your back, your baby can press on certain nerves is what I've learned. try putting a squishy pillow under your belly when you're laying on your side. that helped my back out ALOT.
I'm the same way. lay a pillow or two down under neath your belly then prop up your feet works for me.
You need to get used to laying on your side once your second trimester starts. Laying on your back can cut off circulation of blood and everything.