While flying 30,000 ft I saw a disk shaped lintel cloud dive into another cloud

15 minutes into a flight from LAX  as a passenger at dusk, we had just leveled out above the clouds about 30,000 ft and though the sun was setting, the sky above the clouds was clear and bright.  Movement caught my eye and I turned to look behind me (I have wide periferal vision) and it seemed a small lintel shaped cloud was moving in the opposite direction (south) of the plane.  I mentally checked whether the planes velocity caused me to think it was moving if at all it wasnt, but then it made a westerly turn and started on the same northly trajectory as my plane above the mass of clouds below.  There were two colum shaped clouds that formed a kind of gate and I watched the small moving cloud approach them and told myself, " If this flying cloud goes between those colums I'll know Im not crazy."  It did and began to gain on us. At that point I turned around to see if anybody else was seeing this except me, but nobody in the sparsely occupied plane was looking.  I watched it fly between the coloums and as it approached to a point in a perfect line with my shoulder DIVE into the cloud mass below at aproximately a 30 degree angle and disappeared.