If your BP is normal then you do not have HELPP. My next thought would be that the pain is caused by a combination of chest expansion and relaxin. During your pregnancy your ribcage expands in size. This can put some strain on the cartilage between the ribs. Relaxin causes all the joints and ligaments to relax. One of the many downsides to this is that your muscles may have to work a little harder than they are used to. Another thing that you may wish to consider is your sleeping/sitting position. Staying in one position for extended periods of time will put a strain on your body and this can lead to pain.