Please don't worry, this is incredibly common!
We tend to have this idea that their food intake is supposed to be static all the time and never change (or slowly increase as they get older) but actually that's not really right, it tends to fluctuate, sometimes quite a lot - it can be really hard to gain an idea of what's right/normal because there is such incredible variation, some babies/toddlers seem to exist on a couple of puffs, a bit of leaf they found at the park and air. And others are inhaling three weetabix for breakfast and still demanding a banana. Most are somewhere in the middle, but rest assured that there is a quite astoundingly wide range of what is normal, and it can change even for the same child from one month to the next.
Don't worry. It all passes through. Volume eaten does not correlate with growth or development or sleep or any of it. As long as you are not restricting food (and they don't have dietary restrictions/sensory issues) they do not starve themselves. Sometimes they need more, and sometimes they need less. I have three children and they have spanned the entire range. My eldest completely refused food except for a bite or two a day between the ages of 9 months and 22 months. Of course I worried about it, I kept getting told to cut down breastfeeding, I received all sorts of stupid unhelpful advice actually, and in hindsight I think he was just reacting to the one thing in life he could control, that or perhaps he just wasn't very interested in food. He is now 14 and still eats very little. Of course, he's having a teenage growth spurt so that balances things out and he eats basically a normal amount, but he is never going to be overweight, put it that way!
Children around one year old often drastically reduce their food intake. Their growth slows significantly at this age, so partially, they simply don't need as many calories. They are often distracted by other things, such as learning to walk or talk or other developmental milestones. They are getting molars, which can really change the feeling in their mouth as well as make them feel ropey. This time of year is also shit for viruses, particularly this year, we're still feeling the impact of the lockdowns so there are more viruses about then normal. There's even a theory that it's a kind of protective thing, that toddlers who are just becoming independently mobile often become fussier in order to protect them from ingesting poisonous berries etc back in hunter-gatherer days. I don't know if it's true but I liked reading it at the time!
Children who are breastfed sometimes don't change over to food being the bulk of their calories until closer to their second birthday. IME trying to cut down prematurely doesn't actually help them eat more and is likely to stress you both out. Their growth doesn't suffer if they stay on majority breastmilk so you can just carry on with it. It is annoying, but IME even the ones who go straight onto food before a year and only breastfeed morning and evening (DS2) still wake at night anyway, actually DS1 slept better at night than DS2 between age 12-24 months, so from that perspective, it doesn't matter anyway.
A book that was recommended to me at the time was Carlos Gonzales' My Child Won't Eat. It was out of print then so I never got hold of a copy but apparently it is good. For more up to date resources I like Kids Eat In Color on instagram - a voice of sanity among all the competitive instamums. Their guide to things like portion size and how to count vegetables are really reassuring.
Don't worry. Keep breastfeeding. Keep offering food. Your toddler will eat. I promise. Trust them. You're doing a great job.