Chris,
Ivy and Noah were exactly the same. They went from having full feed of 150mls to drinking between 50 - 100mls.
It is scary but what Trish says is true. If they are having 5 - 6 good wet nappies a day then there isn't a problem.

Somedays now, my two will drink alot and eat less and others it is the other way around.
I still get distressed, especially with young Ivy, who will sometimes have 30mls of a 150ml bottle and then refuse.

I guess they are just like you and me. Do you drinK the same amount fluid every day? I know somedays, I can get to the end of the day and have only had a couple of cups of water and maybe a cup of tea, I just wasn't that thirsty.
Take heart, especially when they are starting solids, I think it is very normal to fluctuate in their intake. Hugs to you.