If you have pain it must not be too good. I agree let your OB or midwife know. I can't lye down on either side because he'll kick nonstop and since he's breech he kicks the h.eck out my bladder! I need to lay face up with like three pillows to be "comfortable" and he'll be still- no kidding! They bully and take over our bodies! :/
When you lay on your back or left side you can reduce blood flow both to baby and to you (this is why some women get dizzy when they do) when baby's oxygen is reduced heart rate goes up and baby can become agitated. Just his way of saying "hey mom roll over"
Babies tend to pick positions they like and he may not like it. But if you are having pain I would bring it up to your doc/midwife, better safe than sorry...
All I've heard that it's better to be on left because they get more oxygen .... not really sure because I'm in left or right and he moves allot:) sorry I wwas not much help...