free space gobbled up - lost 100gb on SSD

I have a MBP running HS 10.13.1

500 gb SSD

my files, system, everything is approx 250 gb.

Using "what size" app worked out at 257gb.

However all of a sudden my free space went down to about 109 gb.

restarted.

ran disk utility from recovery partition.

ran onyx

did the trim command on Time Machine to free up space.

for a short while, free space went upto 220 gb

then later today went down to 109gb

currently its at 184gb

what is going on. ?