Bulging Lines on Walls

Hi, I have 2 lines that seem to bulge out if the wall completely parallel to one another with about a 10cm gap between the on the party wall on my staircase. They span about 5ft horizontally. No cracks but they seem to be getting more prominent. No damp, it's not cold and there is no give in the wall. Can't seem to upload pictures any ideas of what could be causing this? Brick house, no signs of leaks in the loft/roof etc