Congrats on the pregnancy! They say with a second or third pregnancy your body "remembers" how to be pregnant so you can get bigger quicker than with a first pregnancy. I guess it is easier for the uterus to stretch since it has been stretched before.
When I was prego I also had terrible back pain and abdominal aches. I had a fibroid and was considered high risk due to that. The fibroid was the cause of my aches...perhaps you have a fibroid too? They are pretty common actually. Maybe ask your doctor next visit. If sex is painfull I suggest you stop until after the baby comes. I was not allowed to have sex during my pregnancy due to the fibroid, and let me tell you...I didn't mind one bit. Trust me, you can make it a few months without it...and so can your fiance. If something is painful it is your bodies way of telling you something is wrong. Definately bring all this up at your next ob appt. And make sure you ask if you could possibly have a fibroid. It should be easily spotted on an ultasound, but even my doctor missed it for the first few months. Good luck to you. Just tell yourself that your baby is worth all the misery! As soon as you see that little face you will forget all about this!