Bad pain 15+3

So im currently 15+3 and my job is pretty labor intensive and normally 8k step just from work and lots of lifting. And on average 80 miles of driving a day. Im the owner and do not have employees so not much way around it. Anyways... half way through the day my lower back is killing me. I just started getting a belly and have only gained 8 pounds. Its really worrying me what its going to be like when I have a huge belly. Hubby rubs my back almost every night. Would a heating pad during the day help? What do you guys do for back pain in 2nd and 3rd trimesters? Please HELP