saddle position question

I hope you guys can help me! I seem to be rolling forward onto the front of my pelvic bone. I'm not sure if it's an issue with my seat or if there is an issue with the way I'm actually riding. I have lower back problems and I'm wondering - when you ride do you "round" over your lap or flatten your body over your lap. I think I'm flattening but I'm not sure - and it's hard to compare myself to others because it's hard to know what I'm actaully doing? Make sense??

I did have my bike shop check my seat and they said it was tilted slightly - they leveled it out but that didn't solve the problem.

Any advice on this is appreciated!!

update!

I'm going to get a professional fitting tomorrow to see if they can help me with my position on the bike. Before I start putting money into stems and new saddles I want to see if it's my back or the bike iteself.

Any words of wisdom or things I should remember to ask during a professional fitting?? Oh, and I'm going to a different shop this time - just in case.