Hello, I'm wondering if anyone has any ideas or information on what I could be experiencing.

almost a year ago, when laying down I began to notice a feeling of pressure in my lower right abdomen, almost in line with my hip bone. When I feel the pressure if I place my hand there I can feel a large mass in my pelvis/lower abdomen, but this is not present all of the time.
I visited the GP who on examination couldn't feel anything as the mass wasn't present/protruding at the time, but he sent me for an ultrasound, again nothing was found at the time. I've had bowel problems for some months now varying from painful to pass stools resulting in bleeding, to soft mucusy stools more regularly, and this can vary between the two from one week to the next. A few days ago when the mass was present I managed to take some photos and have emailed these to the gp requesting another appointment. 
Today I saw the GP again, where again he examined me to no avail, he's ruled out a hernia which I understand as I have never thought this to be the issue. He agrees that the mass can clearly be seen on the images but as he cannot feel it at the time of examination he feels there's nothing to look into further and I should only come back if the mass starts causing pain.

Im worried that whatever this is could be allowed to worsen before anyone is interested in investigating further. Does anyone have any advice as to what it could be or where I should go from here?